Try to import smaller amount of Thunderbird folders at a time. See post extract from @Gary from the following thread.
Also see alt Thunderbird export / import options you can do further down the thread If that still doesn’t work.
https://forum.emclient.com/t/import-from-thunderbird/62162
“One thing you can try, rather than importing everything at once, is to import selected items a few at a time”.
“So, when starting eM Client, if it prompts you to import from Thunderbird, just cancel that. Then go to Menu > File > Import > Thunderbird > Import selected folders”