monline

There's some sort of conflict between the "Tab Mix Plus" extension and the "Ubuntu Firefox Modifications" extension (the latter is installed by default on Ubuntu). If both extensions are enabled and you've the "Web Developer" extension installed too, the toolbar of the latter seems to disappear from time to time. The solution: disable one of the two extensions (I used to give up the Ubuntu extension), exit Firefox, delete the localstore.rdf file from your profile, start Firefox. Note that this will remove all of your toolbar customizations, thus you'll have to redo them after the cleanup.

Btw. a corrupt localstore.rdf can cause a number of problems and not just toolbar related ones!