Thunderbird is mutton dressed as lamb.
Try importing your Outlook emails into Thunderbird. That should be easy, shouldn't it? Literally MILLIONS of potential Thunderbird users would expect to do this. Well it might sound easy, but when Thunderbird has finished 'importing' you'll find that only the emails that were in the 'personal folders' you created in Outlook (eg for customers, friends, etc) have been imported. Nothing in your Outlook Inbox or Sent folders will have made it across to Thunderbird. Of course they don't bother to mention this in the (non-existent) Help files. You'll only find this out after wasting a lot of time actually doing it. Then you'll search for a solution on Google and find that countless other people have the same problem - year, after year, after year... But still the problem persists and is not addressed, whereas trivial gimmicks, like multi-coloured quote highlighter lines, are cheerfully promoted at great length in what pitifully little official 'Help' actually exists.
Search a bit more and you find horror stories about Thunderbird being unable to import its own 'exported' records. It doesn't actually have an Export function, in fact, so even 'exporting' data has to be achieved by obscure, undocumented workarounds. For example a common requirement might be to transfer your Thunderbird email files to Thunderbird on another computer. Simple, you would think. Just export my files to a DVD, then import them again on the other machine. Prepare to be disappointed...
I'm sure Thunderbird is all terribly clever if you have endless hours to waste coaxing it into a working condition, and if you enjoy disappointments and setbacks. Back in the real world I, for one, just don't have the patience any more to put up with this type of aggravation.
Firefox is a polished and successful application. Thunderbird basks in the reflected glory of its high profile sibling, but it is the runt of the litter. I don't care if it's free and created by dedicated volunteers - it's hyped up to a level it can't justify. It wastes a lot of people's time. Approach with care.