Without saying Bitdefender or any other antivirus product is bad for Thunderbird or that this product may be incompatible / suffer a bug, I would suggest to disable its email scanning feature from within the product, or add an exception for the Thunderbird product and profile folders only, and to uncheck ''Allow antivirus clients to quarantine individual incoming messages'' in the Thunderbird Options > Security panel > Antivirus tab. You don’t really need it (unless you have a habit of clicking any unexpectedly received attachments), and in general has more disadvantages than advantages. See [https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_Performance_Issues this page] for more info. For your issue: it may be solved by disabling a particular feature as above. I cannot imagine Bitdefender to have changed some hidden Thunderbird preferences or even Windows registry entries for opening links (for launching them in your default browser - assuming this is what you refer to.) You can also try what happens if you disable the product entirely (you may need to restart Windows), or (first) what happens if Thunderbird runs in its [[Safe Mode Thunderbird|Safe Mode]]. If Safe Mode helps, a Bitdefender add-on or plugin in Thunderbird (if any) may be the cause, so you can also disable that. Please let us know how this works out.