Every time there is a major update of Thunderbird the extensions I use most stop working. Eventually, months later they are available again. The main one is the export/import profile extension. This is invaluable as the alternative method is too technical. To get round this I have resorted to using the portable version. With this I just copy the entire application as a backup. Perhaps I will stay with the portable version as there is no downside that I can see.
There is a beta version 10.1.0b8 for TB 91, but I haven’t tried the profile import/export feature.