Rooster1 said

I checked tools>options>attachments>incoming and there is no http or https listed under incoming tab. Is there any way I can add them there

They should not be there. Hyperlinks are handles by the operating system, and usually slowness is a direct result of the installed anti virus. Try tuning it off for a bat and try some links and see if it is faster. Degradation in performance of up to 40% is considered normal in the anti virus world.

I tried doing it after uninstalling my anti-virus program (Avast), and it didn't fix the problem. The links are still extremely slow to load - it takes about a minute for them to load in Firefox.

I guess I should expand on my situation a bit. Couple of days ago I had to reinstall Windows10. I backed up my thunderbird profile, and after reinstalling Windows, I activated the profile in Thunderbird. Everything works well -I am able to send and recieve emails - except that the links from thunderbird are really slow to load in Firefox. I didn't have this problem before reinstalling windows and migrating thunderbird profile.

Any other ideas on what could be causing this?

dare I say it, try installing chrome. If that works ok, try installing Firefox again.

Also check the windows defender / Firewall. I noticed in my machine after a recent update that Microsoft had turned on some sort of URL monitor rubbish that only really works with edge, but it slowed things down for me.

Ha, fiddling with the preferences in 'Windows Defender Security Centre' seems to have solved the problem. Thank you very much.