Have you ever found yourselves in a situation with Firefox for Windows where you keep getting the message “Restart Firefox to complete your changes”, even after a restart? It happens if Windows has Offline Files enabled, the Firefox profile resides in an offline folder and you have installed or updated an add-on sensitive to the situation.
It turns out that certain add-ons require the profile folder to be online to install or update properly. Until this happens, they keep asking for a restart to successfully complete installation.
Luckily, the solution to this is simple: start Firefox when the profile folder is online and these add-ons will complete installation properly. If you need to, you can go offline again (preferably after closing Firefox while still online). On next start everything will keep working fine.
If this also happens under different circumstances, please let me know.