I had this recently when doing a presentation. The beamer resolution was bad, so in order to have the whole page visible I did a few Ctrl+- (Zoom out) on the presentated webpage. After Desktop mode, I also wanted to show what the page looks like on Mobile, so I switched to RDM. This totally broke RDM and I had to close the tab and reload the page in a new tab. STR: ==== 1. Go to https://www.mozilla.org/en-US/firefox/ 2. Do a few Ctrl+- so that Zoom is about 50% 3. Switch to RDM using Ctrl+Shift+M ER: === Well, hard to say, despite the bug described in AR I guess the RDM UI should not be zoomed but only the displayed webpage content. AR: === 4. At first I was confused. Both RDM UI is zoomed and not readable, content is zoomed. 5. After a few seconds someone suggested to do a Ctrl+0 to reset the zoom but then only the RDM UI gets back to normal. The webpage content is still zoomed and there is no way to change this. Even reloading does not help. The DPR setting also goes to someting like 0.1 or so, but resetting it to 1 does not help (even if reloading afterwards). 6. Eventually I had to close the tab and reload the page in a new one.

Thanks for the report! I think you have nicely summarized several known zoom issues with RDM, so I'll morph this into a meta-bug to track such things.