Yes, it's safe, but I always recommend doing backups prior to major updates. BACKUP

1. ensure thunderbird is not running
2. enter %appdata% in search window on task bar and press Enter key
3. this should open windows file explorer in Appdata\roaming folder
4. highlight the Thunderbird folder and copy to external media or another place on hard drive

Because 142.0 has been released, I will say that it has been tested. However since it's still pretty new, you might be safer to upgrade to 140.2esr, which has been out longer and has proven to be stable.

Basic functionality of all versions should be good. The thing to watch out for is if your extensions are compatible with newer versions. As I recall, some significant architecture changes were made since ver. 128. There is an add-on called "Add-on compatibility check" that can tell you if your current add-on will work with later releases:

https://addons.thunderbird.net/en-US/thunderbird/addon/addon-compatibility-check/

Before upgrading, I recommend making a backup copy of your profile, in case it doesn't work out and you need to downgrade.

EXCELLENT suggestions! Thanks for adding that. :)

I really only have minor issues with the current version. On one of my Macs, when I am composing a message and I select either a numbered list or a bulleted list, it ignores the selection unless I first type a space character. On another Mac, it behaves as it should.

Another item on my wish list is to be able to "share" my Message Filters from one computer to another. I have no idea where those are stored, but it sure would be nice to be able to copy/paste whatever component holds that data.

Lastly, I find it nearly impossible to delete a faulty email address from the Address Book. It always results in an undeliverable message and I am clueless where that defective address is stored.

Please post a new question detailing your problems with address book https://support.mozilla.org/en-US/questions/new/thunderbird