A chance to do some long-overdue good, actually
In general all it needs, and all that it should be touched for, are security fixes for vulnerabilities that emerge, a very few bug fixes, and one or two config rationalizations.
Beyond that, the only change that is REALLY needed, and this is in common with Firefox, is some semblance of sane memory management. This is in reality a bug fix for a colossal and deeply embarrassing super-bug. I am sick and tired of watching my Thunderbird process in ps grow in RAM use past 1, 2, 3, 4, 5, even yes, 6 GB, instrumental in driving my 16 GB RAM super powerful desktop into memory exhaustion swapping so bad that it cannot be recovered without hitting the power switch.
It gobbles RAM with every message you read, every search you run, every time you switch between accounts. I've repeatedly verified that it hogs down RAM, more and more over time and without any limit whatsoever, even just sitting statically on the desktop collecting new mail.
Firefox with plenty of tabs open shares the same inexcusable RAM hogging.
There is no excuse for this infantile, incompetent level of code design. It is only possible to read one message at a time. Having collected the other messages, the collection code's job is DONE until a new one is opened to be read. All RAM used to collect them should be freed. This is NOT rocket science. Same with Firefox tabs. Only one tab is active and live (or one SHOULD BE so) at a time. Having rendered other tabs, all RAM used to do so should be FREED.
In the case of Firefox, I completely abandoned it for Chrome. The same problem is still there, but not quite as bad, and the entire browser does not get bogged down as horribly just because a bunch of tabs are open.
For Thunderbird, I mostly had to abandon it in favor of Claws because T-bird's behavior got so pathological. I had to give up significant features to do so, but I can leave Claws running for many days at a time, collecting many accounts, and RAM use does not climb at all.
$ ps -C claws-mail,thunderbird -o vsz,rss,comm,etime
VSZ RSS COMMAND ELAPSED
1802716 790736 thunderbird 10:00
1337436 187896 claws-mail 15-10:52:31
(sorry, code and pre tags do not work properly)
Yeah, Claws has been running for 15 days, while I just started Thunderbird 10 minutes ago for the illustration. That Thunderbird RSS will climb to 6,000,000+ (6 GB) if I leave it sitting open a few days.