Can I draw a straight line with the pdf function ?

Hmm, it looks like the drawing tool in the PDF editor doesn't have a feature (like some programs) where you can hold down a key to force a straight line. [[View and edit PDF files using Firefox’s built-in viewer]] That would be a great idea. Mozilla has a product suggestion site at https://connect.mozilla.org/. If you have time, I think it would be helpful to post a request there to get into the current feedback stream. Use the "Ideas" section of the site (at https://connect.mozilla.org/t5/ideas/idb-p/ideas).