Sometimes when I get on firefox my phone has searched the word null. I'm kind of scared

Welcome to Android Central! Where do you see that? Does it show up in your recent search history when you start typing something in the URL bar?
 
Firstly, don't be scared.

Secondly, how do you open Firefox? Maybe you use a icon which is actually a bookmark of this search. Try opening Firefox directly from the app drawer.

If this doesn't help, uninstall Firefox and reinstall it again.
 
It didn't work

What didn't work? Opening Firefox directly from the app drawer or removing and reinstalling it?

You could also try deleting all Firefox's data - go to Settings->Apps and notifications->App info->Clear Data and Clear Cache.
Note: this will delete Firefox settings and bookmarks (but you don't have any bookmarks so that's not a problem).
 

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