Humm, I debugged it a little bit and I think I found the cause for my particular problem:
librejs seems to be changing the contents of the response of http requests that contain html code.
My website uses AngularJS - and as such some snippets of html are retrieved from the server at runtime.
One of such html files is called issuecards.html.
This is what the browser receives from the server without librejs enabled:
And this is the same file retrieved with librejs enabled:
You can probably reproduce this behaviour by pointing firefox to:
Should librejs really be allowed to intercept and change http requests like this?
What is the best way to fix this problem for my website?
Note: I still didn't look at that rootstrikers. Will do very soon. Thanks!