However, the growing use of Firefox means that the hackers will start looking for security holes in it.
Of course. The difference is that Firefox is locked down pretty well. When a new house is built, it's inevitable that thieves will case it looking for ways in. Firefox is the house with its doors and windows locked, and an alarm system set up.
They've been doing that for a long time now, since before Firefox itself existed. Mozilla/Firefox was has been pretty popular among the hacker community.