Looking for opinions on this. Wanted to open up more ways for my PCs to get Firearms Proficiency, and thought weapon master is one way, but the feat kinda sucks.

So my idea is in proficiency in 3 new weapons, and if its a weapon you already have a proficiency with, gain a +1 to attack with it. Thatd not too game breaking is it?

I don't think it's game breaking but it's also pretty unconventional. I can't think of anything else in the game that gives you only a +1 to attack rolls with a specific weapon, and it makes the purpose of the feat less focused.

Giving it +1 STR or DEX (player's choice) is probably enough to make it useful while staying in line with other feat designs. I'd go so far as to say the feat's primary benefit is so weak you could get away with both a +1 STR and +1 DEX bonus without it being overpowered or a no-brainer. It might bother some people that it's giving a total of +2 to your ability scores, but I can think of plenty of situations where you'd still want to pick another feat or put +2 into one ability instead of splitting them 50/50 like that.

Personally I would just switch it to "You gain proficiency with all simple and martial weapons." Presumably if you are taking the feat, you already lack proficiency in a large number of weapons, and switching it to total proficiency is only as powerful as the starting proficiencies of a fighter, paladin, etc. 

You could also tack on an extra ability if you wanted to specify that by taking the feat you gain proficiency in firearms as well, although I am not sure if firearms are already categorized as martial ranged weapons.