My last experience with Tor was somewhat painful due to the sluggish nature of it. If Firefox added Tor as a feature, making Tor "mainstream", could the existing Tor network handle the extra load?
My understanding (I'm no expert on this) is that the more users, the more efficient routing is within the Tor network, in principle making .onion space less sluggish, but if you have a big increase in the number of users with no corresponding increase in provision of exit nodes, then the existing bottleneck for accessing sites outside Tor will get worse.
You need more "nodes" not "users". Tor isn't peer-to-peer amoung "users". You'd need more people to run nodes, which is a service/server you run.
Are you aware that users running the TOR Browser Bundle will normally be a node (i.e., they will provide proxying services to other users/nodes), just as with Bit Torrent clients?
Could exit-node functionality be added to firefox? Make it more peer-to-peer?
There is no technical reason why it couldn't be added. However there is political and ethnical reasons to not add that funcionality. There is debatable legality of running a tor exit node. You don't want "Use Firefox, get a visit to the police" to be a headline people can use why not to use FF.