hello, can you try to replicate this behaviour when you launch firefox in safe mode once? if not, maybe an addon is interfering here...

Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

Hello madperson. I have the same issue when I restart firefox in safemode. Besides, the only add-on I'm using is ad-blocker.

it shouldn't be happening :-/

can you look inside your profile folder for a file that overwrites your ui-layout? go to firefox > help > troubleshooting information, click on profile folder/show folder and close all firefox windows afterwards. a windows explorer window should open up - in there navigate to the chrome-subfolder and look for a file named userContent.css - if it's there please try to delete or rename the file... directly inside the profile folder you can also have a go at renaming/deleting the localstore.rdf file, however it should have already been wiped when you did reset the toolbar customizations.

I've done exactly as you said and the userContent.css file isn't there, although userContent-example.css etc. is. However, I did try renaming/deleting the localstore.rdf and it did not work :S.

this is very strange. can you test it with a new profile too?

Profile Manager - Create, remove or switch Firefox profiles

Creating a new profile worked - the new tab button appeared correctly! What do you recommend?

in this case you could either reset your old profile (which will keep your bookmarks & passwords) or move the most important personal data from your old profile over to the newly created one: Recover important data from an old profile