Please send a screenshot of the error messages you are getting. Please note that this is a Thunderbird Forum. We do not deal with Firefox.

If you were referring to Thunderbird and not Firefox, then:

1. The following link might be helpful: Import settings and data into Thunderbird https://support.mozilla.org/en-US/kb/thunderbird-import

2. "Outlook import requires Outlook to be installed." And Outlook must be the default email client of your operaing system. Besides, the version (32 bit or 64 bit) of Outlook must be the same as the version (32 bit or 64 bit) of Thunderbird.

3. However, even all the above requirements are met, things might not be what you would expect. My experience:

The process of importing from Outlook did take quite a long time, and it seemed that nothing happened, because the progress bar did not show progress. After the process completed, I found that only some of the folders contained emails, and some did not, although they should.

Then I uninstalled Thunderbird, and removed all of the data. Then I installed Thunderbird again, and imported from Outlook again. The result was the same: some folders did not contain emails.

Then I used the following workaround: If an email existed in Outlook but not in Thunderbird, I sent the email to myself by using Outlook, and then I received the email by using Thunderbird.