I did a silly thing: I used Firefox to test the Documentation html page generated by “make html” on NuttX, then after that I closed Firefox, and run again that “firefox index.html”. Until here, it was not a problem, because I could go to: three bars (hambugers) -> History -> Restore previous session. But I forgot do to that and closed Firefox again.

Then when I opened Firefox and asked to restore previous session, it restore only that index.html page. So, before I got so desperate I decided to search on Internet and found a simple solution here:
https://support.mozilla.org/tn/questions/1408206

All I need to do it: three bars -> History -> Recently closed tabs -> Reopen all tabs