Firefox is not working properly on my device.

There isn’t really enough information here to know what’s wrong yet. I’d ask what “not working properly” means first. Is Firefox crashing, not opening pages, running slow, freezing, or having trouble with a particular website?

For a general first try, make sure Firefox and Android are up to date, then restart the phone. Mozilla also recommends clearing Firefox’s cache if the problem continues, but I wouldn’t clear the app’s data unless you know what you’re doing because that can remove local browser information.