Affected versions

• Firefox 75.0b2
• Firefox 76.0a1

Affected platforms

• Windows 7 x64
• Windows 8.1 x64
• Ubuntu 18.04 x64

Steps to reproduce

1. Go to Hamburger menu -> Customize
2. Click on Tab key to access the selection on page.
3. Click on Flexible space button from the toolbar.

Expected result

• The dotted selection appears around the Flexible space button.

Actual result

• The dotted selection doesn't look good, see the attachment.

Regression range
I will come back with regression range ASAP.

Additional notes

• This issue also appear on back button selection. Verified on Firefox 76.0a1 the issue with selection is present, but the Flexible button have different size, it's bigger then on Fx (75.0b2) beta version.

This looks like a visual issue with the indicator of focus, but keyboard focus in this is already a bit of an edgecase, and as long as it's clear where the focus is (which seems to be fine) then I don't think this is high priority.

This issue is not a regression as it is reproducible ever since it was implemented, I verified Firefox 75.0b2 build where the Flexible space button was small and the selected state of button looks bad (see. Selection.png attachment), then on build Firefox 75.0b8 the bigger Flexible space button was reintroduced, but the selected state looks bad here too (see. FlexibleSpace-Selection.png), verified this issue back to Firefox 56.0a1, looks bad on every build.