uninstall-re-install the firefox first or you may try using TUNE-UP UTILITIES, http://www.tune-up.com/download/, probably there's a problem with your registry.
Ganun din sa king PC with AVG free, pero dito sa laptop ko okey naman with 5 tabs open, Bit Defender as AV. Noticed ko kung Tsikot open ko sa FF sa PC ko nagka-crash pero dito sa laptop ko hindi naman. I tried to uninstall then re-installed FF sa PC pero ganun pa din. Try ko uninstall AVG and install BitDefender dun sa PC tomorrow, and see what happens. Pag di pa rin nawala yung bug, reformat ko PC, he he he.