Well before it was a timer. The fire didn't actually do any damage.
So now, the fire does amage at say, 30 seconds of just the fire will kill you. If you add bullets to that, your death time is much lower than 30 seconds in reality.
Rumor has it, though, that the fire damage was significantly decreased to account for this. Additionally, it's been pretty easy to take a tank down without catching him on fire for a long time now, particularly for a well co-ordinated team, so in the grand scheme it probably just means you'll get lit on fire less and die just as frequently without doing anything useful for your team.
So now, the fire does amage at say, 30 seconds of just the fire will kill you. If you add bullets to that, your death time is much lower than 30 seconds in reality.
Rumor has it, though, that the fire damage was significantly decreased to account for this. Additionally, it's been pretty easy to take a tank down without catching him on fire for a long time now, particularly for a well co-ordinated team, so in the grand scheme it probably just means you'll get lit on fire less and die just as frequently without doing anything useful for your team.