I don't know why this isn't core functionality in Firefox and why this extension isn't among Mozilla's "Recommended" either.
This extension is an immediate improvement, because I now have a much easier time understanding source files that I open in Firefox.
Some slight changes for this to be perfect would be to enable autodetect by default and to choose a more contrasty colour scheme by default (Qt Creator Light/Dark maybe) so the highlighted text doesn't fade into the background.

It does the job, although I wish it could work in the source code of web pages.

Overall is excellent ext!
On note. Settings for autohilite and other one are not saved, so I must click 'autodetect' each time.

Thanks for the update to the new framework!