I’m baffled. What if you test on a new profile?
This takes about 3 minutes, plus the time to test a site with targeted links (like startpage.com results).
Inside Firefox, type or paste about:profiles in the address bar and press Enter/Return to load it.
Take a quick glance at the page and make a mental note of which Profile has this notation: This is the profile in use and it cannot be deleted. That is your current default profile.
Click the “Create a New Profile” button, then click Next. Assign a name like Jan2021, ignore the option to relocate the profile folder, and click the Finish button.
Firefox will switch your default profile to the new one, so click the Set as Default Profile button for your regular one to avoid an unwanted surprise at your next startup.
Scroll down to Jan2021 and click its Launch profile in new browser button.
Firefox should open a new window that looks like a brand new, uncustomized installation. (Your existing Firefox window(s) should not be affected.) Please ignore any tabs enticing you to connect to a Sync account or to activate extensions found on your system to get a clean test.
Do the links open normally in the new profile?
When you are done with the experiment, you can close the extra window without affecting your regular Firefox profile. (Jan2021 will remain available for future testing.)