tldr: I prefer the Thunderbird 78 interface over the new one. What are the risks of running this older version of Thunderbird?
I got a new laptop, installed Debian and decided to use Thunderbird 78 because I really cannot get used to the current "we need more white space" interface in Thunderbird versions 102 and 115. I added all my mail accounts and everything seems to work fine.
Is there a higher risk for viruses or malware using v78? (Everything else is latest updated Debian stable).
Should I expect problems sending or receiving mail? (I never used its calendar or chat features)
I tried playing with themes and settings and CSS hacks, but none work perfectly and it broke when upgrading from v91 to v102. I use folders extensively and am kinda addicted to the left side of my screen with about 90 visible folders (that turn bold when a mail arrives).
That said, I completely understand the impulse. I do this a lot nowadays.
Something to keep in mind is authentication methods if you're using Thunderbird for certain accounts. Like just this year Microsoft broke QAuth functionality with Thunderbird, and I'm pretty sure you can't set up QAuth2 on Thunderbird 78 (at least without an addon).
You can probably count on that sort of thing happening more and more.