The userChrome.css holds style rules for modifying Firefox's user interface, right?

I am not trying to do that. I am trying to change the page itself. Normally this works via the userContent.css. But not in this case.

Is this a misunderstanding here?

Note that your Firefox 88.0 version is outdated.

Verify that you meet the System Requirements (GTK+ and GLib) for the current Firefox release.

• https://www.mozilla.org/en-US/firefox/system-requirements/
• https://www.mozilla.org/en-US/firefox/102.0.1/system-requirements/

Is this a page that the extension opens in a tab or a page that opens in a pop-up? Did you check this in the Inspector?

• https://developer.mozilla.org/en-US/docs/Tools/Page_Inspector

You normally use @-moz-document url() or url-prefix(){}

Thank you cor-el!

Yes, the extension opens this page in a tab.

And yes, I checked it with the Inspector ... and could edit and change the elements of the page just fine.

And in the /home/user/.mozilla/firefox/4fgfbksq.default/chrome/userContent.css I used:

@-moz-document domain(moz-extension://285d678c-ecb3-4003-923d-835fc9d8e0bc/reader.html) and also: @-moz-document url(moz-extension://285d678c-ecb3-4003-923d-835fc9d8e0bc/reader.html) and also: @-moz-document url-prefix(moz-extension://285d678c-ecb3-4003-923d-835fc9d8e0bc/reader.html)

... for instance followed by a simple:

{

   div#toolbar {
   background-color: #D09595;
   }

}

But in all 3 cases: No change of color. Am I missing something here?

I tested on one of my pages and this worked. I did use !important which sometimes is needed to override rules that are considered to have a higher CSS specificity:

@-moz-document url("moz-extension://5cfb8e4d-ab5d-48d2-ac71-8433ee9b62cd/logaddnew.html"){ html {background-color: #fed !important;} body {background-color: transparent !important;} table#CTLog {background-color: #ffc !important;} }

(Those are not recommended colors, just for testing.)

Halleluja!

Thank you guys so much! And especially you, jscher2000! It has indeed been the missing "!important"!

You guys are wonderful! ... and I have so much still to learn!

:-)