Very sure this will not be useful to you, but I tell you my experience about it. Some time ago I was in a dilemma like this, and I realized that the time I spent on this issue with User Agent old version was just too much.
A reader of my content can not imagine how much work I have to do to make my content available to them
And sure there are people with old computers (or cell phone) that they would like to reach my content, but my math is returning some numbers to explain that the time spent is not justified
I am of those who think I do not want to be popular, I do not want a log file with 10,000 visits in one day. Instead I want 3 hits; they came, read, and left.
For you to know, FF does not enter, if not version 9 and 10