I’m affected by this too, in Ubuntu 20.04 with Firefox 83.0+build2-0ubuntu0.20.04.1.

Tested nightly (85.0a1.en-US.linux-x86_64) with a clean profile, and the issue remains.

Steps to reproduce

1. Start nightly with ./firefox -no-remote -profilemanager
2. Create a new profile, select it and click Start Nightly
3. Open these three tabs (replacing the first-run tabs currently open):

• https://twitter.com/i/lists/222412094
• https://twitter.com/uusijani/
• https://twitter.com/i/lists/1245729655862280192

4. Open preferences, set Home to Custom URLs…, select ”Use current pages”
5. Close the window
6. Start nightly again with ./firefox -no-remote -profilemanager, select the previously created profile

What happens

None of the three tabs load any content. The firefox process also doesn’t end after closing this window with the non-working tabs, I have to Ctrl-C out of it.

Further info

Upon subsequent starts, Nightly does appear to correctly load the tabs, whereas for my main Twitter profile and FF 83.0 no amount of restarts seems to help; instead I have to open another tab and open m.twitter.com in it, then click on the home button to have the home tabs load (and close the first, still stuck set of home tabs).