I was running Thunderbird-68.12.0 and Firefox-68.12.0esr. As you can imagine, lots of website don't work properly, for me, company payroll site, income tax site, old school email, and others. So, I finally decided to bite the bullet and upgrade.
Main problem I encountered was Firefox and Thunderbird didn't use my old data. I have shit loads of account settings, filter rules, and emails. After many 4 letter words, I managed to figure out how to configure Firefox and Thunderbird to use my old profiles.
thunderbird --ProfileManager
firefox --ProfileManager
Great tip, William! Thanks.