Yep, she's done...

It's fairly common for the synchros to come apart in the Xterra/Frontier transmission. I'd say if you actually lost first gear it's probably had problems for awhile and there is likely more damage done inside too as a result of all the little teeth being ground up in there... bearings might be bad, etc. And a synchro is nothing simple; remember the transmission has to be removed and taken apart even if that's all it is!

At the very least it needs a partial rebuild. I don't know what repairing costs, but a used one from the junkyard will run you anywhere from $500 - $1000 depending on mileage. I think I paid $750 for mine. You're looking at another $1000 or so to have it installed at a dealer. Make sure you use the proper gear oil in the new or rebuilt one... that will probably help. I can almost guaranty the dealer won't put the right stuff in when they replace it (mine didn't), so you might have to do it yourself. BTW, did you ever change the oil in the oild one? You should have had teeth coming out with the oil...

And no, the 6 speed doesn't match up.