I have a custom piece of Javascript which I would like to run on every web page from specific domains, or perhaps simply on every web page.
(If you are wondering: it is not malicious. It allows to display formulas by using MathJax.)
Is that possible? I tried including it in userContent.css, that of course did not work.
A simple Greasemonkey script I tried did not insert it. Is it because of the security precautions? (Which would be very logical).
Still, there should be a way to do it on the machine I physically control, by changing something in Mozilla chrome directory, shouldn't it?
Anyway, how can I do this for myself?
Greasemonkey should do what you want. If it's not working either it's not being applied to the correct domains, or the code contains some sort of bug! (I personally use grease monkey on stack overflow to make some changes to the answer area).
Try placing some alerts() within your code to ensure that your grease monkey script is executing as intended.