diff --git a/plinth/modules/matrixsynapse/__init__.py b/plinth/modules/matrixsynapse/__init__.py
index 46f917374..cdc35df3d 100644
--- a/plinth/modules/matrixsynapse/__init__.py
+++ b/plinth/modules/matrixsynapse/__init__.py
@@ -33,7 +33,12 @@ _description = [
     format_lazy(
         _('Matrix Synapse needs a STUN/TURN server for audio/video calls. '
           'Install the <a href={coturn_url}>Coturn</a> app or configure '
-          'an external server.'), coturn_url=reverse_lazy('coturn:index'))
+          'an external server.'), coturn_url=reverse_lazy('coturn:index')),
+    format_lazy(
+        _('<strong>Note:</strong> This app receives frequent feature updates. '
+          'It can only be installed if frequent feature updates is enabled in '
+          'the <a href="{upgrades_url}">Software Update</a> app.'),
+        upgrades_url=reverse_lazy('upgrades:index')),
 ]
 
 logger = logging.getLogger(__name__)
diff --git a/plinth/modules/upgrades/__init__.py b/plinth/modules/upgrades/__init__.py
index be4bf2d80..f573ecdcd 100644
--- a/plinth/modules/upgrades/__init__.py
+++ b/plinth/modules/upgrades/__init__.py
@@ -55,7 +55,7 @@ class UpgradesApp(app_module.App):
 
     app_id = 'upgrades'
 
-    _version = 19
+    _version = 20
 
     can_be_disabled = False
 
@@ -212,6 +212,7 @@ def setup_repositories(_):
     """Setup apt repositories for backports."""
     if is_backports_requested():
         privileged.activate_backports(cfg.develop)
+        privileged.activate_unstable()
 
 
 def check_dist_upgrade(_):
@@ -370,7 +371,7 @@ def can_activate_backports():
     if cfg.develop:
         return True
 
-    return not utils.is_distribution_rolling()
+    return not utils.is_distribution_unstable()
 
 
 def can_enable_dist_upgrade():
diff --git a/plinth/modules/upgrades/privileged.py b/plinth/modules/upgrades/privileged.py
index e94ae9e21..ad107cdfa 100644
--- a/plinth/modules/upgrades/privileged.py
+++ b/plinth/modules/upgrades/privileged.py
@@ -17,6 +17,11 @@ from . import distupgrade, utils
 logger = logging.getLogger(__name__)
 
 BACKPORTS_SOURCES_LIST = '/etc/apt/sources.list.d/freedombox2.list'
+UNSTABLE_SOURCES_LIST = pathlib.Path(
+    '/etc/apt/sources.list.d/freedombox-unstable.list')
+
+UNSTABLE_PREFERENCES = pathlib.Path(
+    '/etc/apt/preferences.d/50freedombox-unstable.pref')
 
 AUTO_CONF_FILE = '/etc/apt/apt.conf.d/20auto-upgrades'
 LOG_FILE = '/var/log/unattended-upgrades/unattended-upgrades.log'
@@ -52,6 +57,52 @@ Explanation: bug report 1099755.
 Package: samba-ad-dc
 Pin: release *
 Pin-Priority: -1
+
+Explanation: Make matrix-synapse package and its dependencies installable from
+Explanation: Debian 'unstable' distribution.
+Package: matrix-synapse
+Pin: release n=sid
+Pin-Priority: 200
+
+Explanation: matrix-synapse depends on python3-python-multipart
+Package: python3-python-multipart
+Pin: release n=sid
+Pin-Priority: 200
+
+Explanation: matrix-synapse recommends python3-pympler
+Package: python3-pympler
+Pin: release n=sid
+Pin-Priority: 200
+'''
+
+APT_PREFERENCES_UNSTABLE = \
+    '''Explanation: This file is managed by FreedomBox, do not edit.
+Explanation: De-prioritize all the packages from Unstable distribution.
+Explanation: The priority of packages in *-backports will be set to 300.
+Explanation: Prioritize unstable lower than packages in backports.
+Package: *
+Pin: release n=sid
+Pin-Priority: -100
+
+Explanation: The priority of packages in *-backports will be 100 by default.
+Explanation: Prioritize them higher than unstable packages.
+Package: *
+Pin: release n=trixie-backports
+Pin-Priority: 300
+
+Explanation: The priority of packages in *-backports will be 100 by default.
+Explanation: Prioritize them higher than unstable packages.
+Package: *
+Pin: release n=bookworm-backports
+Pin-Priority: 300
+'''
+
+APT_UNSTABLE_SOURCES = \
+    '''# This file is managed by FreedomBox, do not edit.
+# Allow carefully selected updates to 'freedombox' from unstable.
+
+deb {protocol}://deb.debian.org/debian unstable main
+deb-src {protocol}://deb.debian.org/debian unstable main
 '''
 
 
@@ -243,6 +294,35 @@ def activate_backports(develop: bool = False):
     _check_and_backports_sources(develop)
 
 
+@privileged
+def activate_unstable():
+    """Setup apt sources for unstable distribution and de-prioritize it.
+
+    Select packages will be made installable from unstable.
+    """
+    # Operation already performed, don't write to files unnecessarily.
+    if UNSTABLE_SOURCES_LIST.exists() and UNSTABLE_PREFERENCES.exists():
+        logging.info('Skipping already added unstable sources.')
+        return
+
+    # If the distribution is already 'unstable', default sources.list already
+    # contains sources for 'unstable'. Also, don't de-prioritize the primary
+    # set of packages.
+    if utils.is_distribution_unstable():
+        logging.info(
+            'Skipping adding unstable sources for unstable distribution.')
+        return
+
+    protocol = utils.get_http_protocol()
+    if protocol == 'tor+http':
+        logging.info('Package download over Tor is enabled.')
+
+    logger.info('Adding unstable sources to apt.')
+    sources = APT_UNSTABLE_SOURCES.format(protocol=protocol)
+    UNSTABLE_SOURCES_LIST.write_text(sources)
+    UNSTABLE_PREFERENCES.write_text(APT_PREFERENCES_UNSTABLE)
+
+
 @privileged
 def start_dist_upgrade():
     """Start dist upgrade process.
diff --git a/plinth/modules/upgrades/templates/backports-firstboot.html b/plinth/modules/upgrades/templates/backports-firstboot.html
index 898a2d23c..ee9b01877 100644
--- a/plinth/modules/upgrades/templates/backports-firstboot.html
+++ b/plinth/modules/upgrades/templates/backports-firstboot.html
@@ -14,11 +14,11 @@
     {% blocktrans trimmed %}
       Frequent feature updates allow the {{box_name}} Service, plus a very
       limited set of software, to receive new features more frequently (from the
-      backports repository). This results in receiving some new features within
-      weeks, instead of only once every 2 years or so. Note that software with
-      frequent feature updates does not have support from the Debian Security
-      Team. Instead, they are maintained by contributors to Debian and the
-      {{box_name}} community.
+      backports or unstable repository). This results in receiving some new
+      features within weeks, instead of only once every 2 years or so. Note that
+      software with frequent feature updates does not have support from the
+      Debian Security Team. Instead, they are maintained by contributors to
+      Debian and the {{box_name}} community.
     {% endblocktrans %}
   </p>
 
diff --git a/plinth/modules/upgrades/templates/upgrades_configure.html b/plinth/modules/upgrades/templates/upgrades_configure.html
index a78eef30e..3609c8dfd 100644
--- a/plinth/modules/upgrades/templates/upgrades_configure.html
+++ b/plinth/modules/upgrades/templates/upgrades_configure.html
@@ -68,11 +68,11 @@
       {% blocktrans trimmed %}
         Frequent feature updates allow the {{box_name}} Service, plus a very
         limited set of software, to receive new features more frequently (from
-        the backports repository). This results in receiving some new features
-        within weeks, instead of only once every 2 years or so. Note that
-        software with frequent feature updates does not have support from the
-        Debian Security Team. Instead, they are maintained by contributors to
-        Debian and the {{box_name}} community.
+        the backports or unstable repository). This results in receiving some
+        new features within weeks, instead of only once every 2 years or so.
+        Note that software with frequent feature updates does not have support
+        from the Debian Security Team. Instead, they are maintained by
+        contributors to Debian and the {{box_name}} community.
       {% endblocktrans %}
     {% endif %}
   </p>