After using ... and liking ... your Firefox browser, and setting it as my default browser on all my devices for many years now, just today I noticed while in my Firefox settings a link to download your free Thunderbird email app on my computer. I thought it might be nice to have a single app where I could add ... and view ... both my Hotmail (Outlook) and Gmail. After only 5 or 10 minutes of looking at it, and trying to figure out how to use it, I wasn't the least bit impressed with the looks nor the functionality of it.
Needless to say I deleted all my profile information and uninstalled the app, because both my free Outlook and Gmail desktop apps not only look better but also have much better and easier functionality. Maybe it's just me, but if something doesn't appeal to me right away, and isn't easy to use, then I usually don't waste much time and energy on it.
At least you do have a better browser that I like to use much more so than any other available Windows browser that I've ever used. Maybe after as many years as Firefox has been around your email app(s) might evolve into something much better also? Perhaps time will tell ... if I have that much more time!