gmail in FFP 3.6.16 not opening

I have a FFP 3.6.16 and don't want to upgrade due to previous upgrade issues and old laptop with XP. Gmail stopped supporting this version , no longer opens and I don't want to open mail in other browsers. Anybody knows a work-around for this? If not possible, can I download a new version on the same SD card and use it only to open gmail on the same laptop or it's better to download to another card? I get my mail in TBP and need to open gmail only to make g-voice calls. Any TB or FF add-ons to make those calls without opening gmail? It would eliminate the problem, thanks