try that command in your terminal:

flatpak override --user --filesystem=xdg-cache/mozilla:ro org.mozilla.firefox

After running this command, restart Firefox. The "Tabs from other devices" and Firefox View menus should now be able to connect and display your synced tabs without the error.

Thanks. I tried that but it resulted in the flatpak firefox not being able to boot. If I set the permission to read and write I am able to boot it again I still have the "Check your internet connection..." error.

It's possible that the Flatpak version of Firefox doesn't have the necessary permissions to communicate with your system's network or with other programs.

Install Flatseal, is a graphical utility for managing Flatpak permissions. You can install it from Flathub by running flatpak install flathub com.github.tchx84.Flatseal in your terminal.

Adjust Firefox Permissions: Open Flatseal and select Firefox. Look for a "Filesystem" or "Network" section and ensure that Firefox has the necessary access. You might need to experiment with different settings, but a good starting point is to ensure that it has access to your home directory and network connections.

Thanks for your support and insight George. I wasn't able to resolve things after playing around with different permissions but it sent me on a journey that ultimately brought me to a workaround.

Briefly what didn't work in Flatseal:

• enabling device shared memory (device=shm)
• enabling allow application shared memory (allow=per-app-dev-shm)
• adding xdg-cache/mozilla to filesystem other files

Using Flatseal I was able to give permission to the Firefox flatpak to use my home directory in a similar manner to this post I saw. First I did this by enabling all user files (filesystem=home) but later I limited the scope by adding /home/[username]/.mozilla/firefox/ to filesystem other files. That allowed firefox to create and use a profile folder in that folder along with other firefox profiles. Then I copied the session store data in the Firefox deb profile into the profile folder for Firefox deb. This made all the tabs open in that deb session open in the flatpak version.

Still can't view the synced tabs sidebar nor the Firefox view menu, but I was able to get all the tabs I wanted open. If there are any other suggestions I'm happy to try them out.

The error message you're seeing in the Flatpak version of Firefox is likely due to the application's inability to access the necessary network or system services required for Firefox Sync.

1. Verify Firewall/Proxy Settings Even with a Flatpak application, a firewall or proxy on your system could be blocking the connection.

Temporarily disable your firewall: This is a good way to test if it's the culprit. If the sync works with the firewall off, you'll need to create a rule to allow Firefox through.

Check proxy settings: Ensure that your system's proxy settings are not interfering. You may need to configure the proxy within Firefox itself, as Flatpak apps might not inherit system-wide proxy settings.

Although Flatseal allows you to manage permissions, there might be a specific permission related to networking that's being overlooked.

2. Network Access: In Flatseal, under the Firefox application, double-check the "Network" section. Ensure that "Allow network access" is enabled. This is usually on by default, but it's worth verifying.

3. Check for Conflicting Profiles

Your successful workaround of copying the sessionstore.jsonlz4 file from the Debian profile to the Flatpak profile suggests that the Flatpak version's profile might be misconfigured for sync.

Create a new, clean Flatpak profile: As a test, try creating a new, empty Firefox profile within the Flatpak version. Log into your Firefox account from this new profile. See if Firefox View and the synced tabs sidebar work correctly. This would help determine if the issue is with your original Flatpak profile or the Flatpak sandboxing itself.

How to create a new profile: In the terminal, use the command flatpak run --command=firefox org.mozilla.firefox -P to launch the profile manager. From here, you can create a new profile.

You noted that flatpak override --user --filesystem=xdg-cache/mozilla:ro org.mozilla.firefox prevented Firefox from booting. This is a crucial clue. The xdg-cache directory is where applications store temporary cache files. By setting the permission to read-only (:ro), you likely prevented Firefox from writing necessary cache files, causing it to fail on startup.