Is it safe to download mozilla firefox?

Is it safe to download firefox on to my laptop, i already have a security protection thing but i don't want to download it and then it all crashes and goes funny.. Please tell me if it is safe to download it whilst having another security protection on.I want to download it to beat my dad at bejewelled blitz lol.
Update: i have downloaded it but now how do u make it go in to cheat mode
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