User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:64.0) Gecko/20100101 Firefox/64.0 Steps to reproduce: 1) install add-on like NelliTab https://addons.mozilla.org/en-US/firefox/addon/nellitab/ 2) Select a dark theme in add-on settings 3) Open a new tab Actual results: There is a huge white flash before the extension is loaded. Expected results: No white flash like its made in Vivaldi Browser (You can install NelliTab as well there for testing). Currently there no options/solution or workaround for extension developers to reduce the white flash in Firefox. Strange is if i set the background in UserChrome.css e.g. to "red", then i see first "white", then "red" and at the end the extension background. White flashing is reduced but still there. What Vivaldi is you doing differently?

I am wrong. In Vivaldi it depends on selected theme. Thats why it could be duplicate of other already discussed issues here. Sorry.

Found the difference: - Vivaldi: changing "#main > .inner" in custom.css works / no white flash - Firefox: changes in userChrome.css of Firefox take effect, but to late => white flash.

Hi Thomas, Thank you for taking the time to add this report and for looking into the cause of it. I've managed to reproduce this issue as described above on all the latest Firefox versions and all OS-es (Mac OS X, Windows 10 and Ubuntu 16.04). However, this is a known issue that has been reported a while back (1438143) and has not yet been fixed. I'll mark this as a Resolved:Duplicate of the mentioned issue.