Perhaps odd, but I've had the opposite experience.

I switched to Thunderbird at the same time I switched to Linux; after some tweaking, I was mostly happy with it.

Then, in some major version, it started frequently freezing up for thirty seconds at a time. Authoring emails in it became impossible. I could not downgrade it because it was version-locked with some system libraries (libnss OSLT). I could not debug it because that was blocked by a bug filed on Bugzilla ten years ago with no fixes or workarounds.

Then, in another major version, they killed XUL extensions.

I still hope to stop depending on Google services some day, but at that point I would probably switch to or write some webmail with a similar-enough UI to Gmail.