How to install an export format in Zotero -

i using zotero plugin in firefox on ubuntu12.4 machine.

since last week, bibtex format export seems have disappeared options in preferences. have absolutely no idea why , don't care why. want since worked before. looking explanation on how reinstall option , on format. tried desactivate , reactivate zotero plug-in did not work.

thanks help

well, resolved problem refreshing firefox, i.e. restoring initial state, adding required add-ons , works fine again. suspect upgrade version 39.0 may source of trouble.