I've noticed the same sort of "degradation" when having large PDFs open as well. I have to refresh the "page" (the PDF) to be able to search for something and find that something that I know is in the PDF. Small F5 issue once you know about it cause it remembers where you are in the PDF.
My guess is RAM management/garbage collection going on in the background that needs to whitelist a few things or have some additional fancy stuff going on to not allow for memory leaks but allow for 2 hour phone calls and searchable PDFs after they've been open for some time (and possibly with a 10+ tabs open).
Definitely file a bug. I would imagine both these issues are known about and have a similar root cause.
My guess is RAM management/garbage collection going on in the background that needs to whitelist a few things or have some additional fancy stuff going on to not allow for memory leaks but allow for 2 hour phone calls and searchable PDFs after they've been open for some time (and possibly with a 10+ tabs open).
Definitely file a bug. I would imagine both these issues are known about and have a similar root cause.
I love Firefox otherwise.