Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
How old is your tablet please?
As for Firefox - it will not be there, if you reset the tablet to factory defaults.
(After factory defaults resets, the tablet may have to be configured basically, for internet access for instance, before other things will work. Factory defaults goes all the way into the settings and setup, remember, for network connections.)
Get Firefox again, from an authorized source, the original source that is:
after making sure that your internet connection or Network Settings are OK and you are connected.
Let me know how that goes, I hope to help you through to a solution to this problem.