If Firefox tabs have become all blank
It may happen, that suddenly all of your Firefox tabs —any web page open in any tab whatever— become blank. You can activate tab by tab all those pages, and still aren’t fixed, they remain just blank.
This strange situation may have appeared after you started downloading a file.
It is very easy to gain back all those pages loaded in your Firefox tabs: just open a new tab. Then activate the next or previous tab and you will see that it is not blank anymore!
Blank tabs are fixed even if you immediately open and close a new tab (you don’t need to leave it open). Enjoy!