@ogra Thanks for your insight about the unability to build TB and FF snaps because of the missing i386 packages left in the archive.
I was about to give up on having a current FF and TB on 16.04 LTS 32 bit. And to give up on Xenial all together and also lose the guest-account feature that was broken after 16.04. In Xenial TB and FF did get ESM security patches, but the versions were stuck at TB v68 and FF v88. For FF sync and TB shared IMAP folders and shared Calendars, the versions of the synced devices shouldn’t be apart too much. Effectively still rendering FF and TB obsolete.
Then I stumbled upon this article about adding a PPA to unattended-upgrades .
It mentions a PPA that does offer current TB and FF builds even for Xenial (16.04 LTS) i386 (32 bit) Ubuntu. I tried it and it works!
I now have the latest versions running on Xenial 32 bit (i386):
I’m planning to switch to the firefox-esr build for stability reasons, that’s also in the same PPA.
I’d like to thank the folks that are maintaining the PPA at https://launchpad.net/~mozillateam/+archive/ubuntu/ppa for keeping 16.04 viable and from abandoning Ubuntu on my aging, but working hardware.