You will need to go to Thunderbird — Software made to make email easier. — Mozilla or preferably Thunderbird — Software made to make email easier. — Mozilla which is harder to find than you might think.
There are some useful Thunderbird related links at Useful Thunderbird Links - Thunderbird - Mozilla Discourse which are well worth checking out.
See Mozilla Profiles for details.
It is worth learning some keyboard shortcuts, especially as your hands will be on the keyboard for typing anyway, so check out Keyboard Shortcuts TB | Thunderbird Help, which will detect your operating system and show appropriate keystrokes.
There is a nice article on message filters at Mozilla Thunderbird Filters Guide – J T Response Limited which looks good.
It is well understood, or should be, that e-mail is not secure. However e-mail encryption is not easy, although it is getting easier... see Introduction to End-to-end encryption in Thunderbird | Thunderbird Help for some help on this. The mobile version of Thunderbird also supports encryption much better than it did.
Some people prefer to use other approaches to sharing encrypted messages, like Signal.
You might be running the 32-bit version of Thunderbird and want to move to the 64-bit version. I have done this but first I did some tests on Windows Sandbox to make sure I understood how it worked. My starting point was running Thunderbird 32-bit, which on Windows is installed in "C:\Program Files (x86)\Mozilla Thunderbird" and I wanted to get to the 64-bit version, there are two options:
Both options work perfectly and no data or anything is lost. The advantage of doing a re-install is that you have the 64-bit Thunderbird installed in "C:\Program Files\Mozilla Thunderbird", if you do the upgrade you have 64-bit software in a directory that suggests it is 32-bit.