Syntax error “illegal character” in Firefox but not in Chrome

I get the following error but only in Firefox on loading an ASP.NET page:

SyntaxError: illegal character 1 MicrosoftAjax.js:1

If I try to debug this and load the source of MicrosoftAjax.js from the Firefox console I only get garbage characters which makes me guess this is some kind of encoding issue. However, I never get this issue in Chrome. I don't believe this is actually related to the MicrosoftAjax.js source file.

If I do a reload (F5) of the page in Firefox all JavaScript is loaded correctly and the page runs as expected.

Maybe someone already had a similar problem? I spent many hours looking for a solution with no luck.