We're sorry to hear about this. If this was a device you really don't trust, you should change your Firefox Account password, and change any passwords saved in Firefox.
Jo nam luto to słyšaś. Jolic to jo rěd było, kótaremuž napšawdu njedowěriśo, wy by měł gronidło swójogo konta Firefox a gronidła, kótarež su w Firefox skłaźone, změniś.