Currently, in 8.6.x there are 4 instances of @-moz-document url-prefix() {} being used to supply Firefox only css. However, as of June 26th, 2018, with the release of Firefox 61, that rule will no longer be supported. Already, unsupported in dev/nightly/beta, which is why I noticed it since #2886904: display: block for details/summary hides drop arrows in Firefox (normalize.css update) didn't work. The reasons for removal are security/CSS injection/exfiltration.
Three of the instances, are related to a fieldset width Firefox bug that was fixed over 2 years ago. These should probably be removed when #2390621: [policy, no patch] Discuss aligning Drupal's browser support with jQuery 3's. Decide on exceptions. is finally resolved.
Suggested Fix
Using _:-moz-tree-row(hover), .selector works in all Firefox versions since 3.5 (as far as I can tell, didn't actually test all the way down to there.
Testing
Testing the change related to #2886904: display: block for details/summary hides drop arrows in Firefox (normalize.css update) is easy:
- Using Firefox 61 (or a beta/dev release since 59)
- Navigate to admin/config/system/site-information
- Confirm that the arrow shows on the summary/details element and toggles on opening/closing the element.
Functional testing the changes related to fieldset width would require a very old Firefox version and I believe it can be skipped as long as a code review shows the changes are equivalent.