@JohnJ :-
Welcome to Linux.org!
I, too, have always used T-Bird, even back under XP before I went all-Linux in 2014. Never liked Outlook; it was untidy, and always prone to issues.....with T-Bird, it's always been plain sailing.
Along with assistance from a few others, I've developed an entire range of 'portable', self-contained applications that work very well for us here in 'Puppy' Linux (an original, Aussie-developed distro!)
For my final, annual re-install of XP in 2013, I took a totally different route. I'd recently discovered the delights of
PortableApps.com, the Windows portable app website.....so instead of installing stuff as usual, I kitted-out XP with nothing BUT portable versions of everything. Unbelievably, it was the best year I'd ever had under XP.....almost totally glitch-free, because those portable apps didn't write to the Windows registry. They all had their own, self-contained mini-registry
within the app itself.
Ever since moving to Linux, and finally Puppy itself, I'd been wanting to recapture that ease of operation.....where all config files and everything were 'self-contained' - not 'on the system' at all - to the point where you could even run apps from a flash drive if you wanted to.
I started off with Firefox and Pale Moon, followed rapidly by T-Bird itself. These worked so well, I got the bit between my teeth (some 5 or 6 years ago), and ever since I've been 'portabilizing' everything I can!
I haven't stopped yet.....
Mike.