I can modify the "Accept" header in outgoing HTTP requests in Firefox 129, so it still works.

It would be nice if syntax errors in the rules would be shown.

Nice!, works greatly. but I would need two things:
- able to put shortcut on bar to enable/disable the inserts
- add check per url key basis. then we could put /api/xpto/ as url key. so it would be active only when url contains this text

Example rules:
example​.com;/api/xpto/
.*regexp\d+

thank you!

That’s a nice tool with a friendly interface (which are so rare nowadays), but it’s worth stating clearly in the description, that it is able to modify *only request* headers. (Even better — to make it possible to modify response too, of course. ;-)