[Note]:

• This issue is intermittent and at this point we don't know what triggers it
• This issue doesn't occur on OSX 11.2.3

[Affected versions]:

• latest Nightly 89.0a1 2021-03-30

[Affected platforms]:

• Win 10 64-bit
• Ubuntu 18.04

[Preconditions]:
The following prefs are enabled in about:config:

• browser.proton.enabled
• browser.proton.toolbar.enabled
• browser.proton.urlbar.enabled

[Steps to reproduce]:

1. Go to about:preferences and check the option to "Restore previous session"
2. Open a few about: pages and observe the address bar chiclet
3. Restart the browser

[Expected result]:

• Address bar chiclet stays the same after restart

[Actual result]:

• Address bar chiclet is replaced by the magnifying glass, the page needs to be refreshed for it to be shown again

[Regression range]:

• Due to the fact that the issue is intermittent we can't provide a regression range at this point.

[Additional notes]:

• There are no errors in browser console

I managed to reproduce this as well, on about:support.
One thing to note is that initially the chiclet appears but is quickly replaced by the magnifying glass icon.

It looks like more a focus problem. Considered we only changed the styling here I think it's not a Proton regression.

I haven't encountered this on Firefox 89 beta 1 to 11, so I'm closing as RWFM for now.