For the record...
I recently switched to Linux Mint, and ever since then I right-clicks in GMail on my firefox had stopped working.
Disabling all the labs feature fixed the problem, so I started hunting for the specific offending labs feature.
In the end it turned out to be the 'Mouse gestures' feature. Since I don't use that feature at all, it was easy to just turn it off, and voila, my Firefox+GMail is back to normalcy.