hello Jarfulous, this might happen when firefox is not able to properly save preferences in its profile folder due to a user.js file. in order to resolve it, please try this: click the menu button ≡ and then click help ? > troubleshooting information > profile folder - "show folder". then a new window will open up. in this window look out for a file named user.js, delete or rename this file and restart firefox afterwards.

That did the trick, thank you!