So, I tried installing the default firefox 3 using this guide: https://help.ubuntu.com/community/FirefoxNewVersion
After the install all spaces in firefox were replaces by tabs. (se image below)
I removed the new install, but when I ran the default mint firefox the tabs were still present.
Reinstalling didn't work, so I tried resetting everything by uninstalling the firefox packages (firefox, firefox-3.0, firefox-3.0-branding etc.) and renaming all mozilla and firefox folders I could find. Didn't work.
I have no idea what to do. Since I didn't manage to install firefox "from scratch" I don't know what to do.
Hopefully someone might have some ideas.