Some mailboxes didn’t transfer, and some didn’t have all of its original messages. I attempted the Apple mail Thunderbird import where I could point it to where my mailboxes were physically located on my hard drive and it tried to suck everything up, but it failed. I had already used it on my iPhone and iPad, so it was time to switch. I finally decided to move all my mail over to Apple mail.
I love supporting Mozilla–and still use Firefox–but Thunderbird had to go. After the last Thunderbird update there were so many bugs in the software, I just couldn’t deal anymore.