profile folder/chrome/userContent.css

Thanks but no, could not be this since all that userContent.css has is one rule to ensure body text is normal (as set in users preferences). There would surely have to be a really big stylesheet somewhere that tells the browser, in the absence of author rules, many other things, like for instance what colour the text should be, how big various level headings are. This sheet operates even when there is no author styling and likely when one disables styles altogether.

No one in the community knows the answer of where the basic stylesheet is?

Use these URIs to see them in a browser tab:

resource://gre-resources/html.css
resource://gre-resources/quirk.css

You can find them in the omni.jar archive in the Firefox installation folder.

• omni.jar/chrome/toolkit/res

Yes, thanks, the following gets them:

"resource://gre-resources/html.css resource://gre-resources/quirk.css"

but

"You can find them in the omni.jar archive in the Firefox installation folder. omni.jar/chrome/toolkit/res"

No idea how to see this latter, how do you get to see what is in omni.jar?

The file omni.jar is a ZIP archive in the Firefox installation folder that stores a lot of the extra (chrome) files that Firefox uses. In previous Firefox versions those files were separate files on disk, but those files were moved to one large ZIP archive that has an optimized file order to load the files in it as quickly as possible during the start up. If you want to inspect files in that archive and your file manager doesn't allow access directly than make a copy of the file and add a .zip file extension.

You can open that archive file and browse to the /chrome/toolkit/res folder.

Thanks, yes, that's good.