There are a wide variety of furbolg on the continent of Kalimdor. Some tribes have been tainted by the fel energies released when the Burning Legion attacked Felwood and marched on Mount Hyjal itself, while others have managed to stave off this infection to a greater or lesser extent. The only two currently extant tribes of Kalimdor furbolg known to be uncorrupted are the Stillpine of Azuremyst and Bloodmyst Isle and the Timbermaw of northern Felwood. The current status of the Barkskin furbolg once found within the barrow dens of Mount Hyjal is as yet unknown.
Greatly outnumbered by their corrupted kin, these tribes still attempt to live in adherence to the old ways and have even gone so far as to make alliances with outsiders. The Stillpine reach out to the refugee draenei who come to inhabit their isles, going so far as to even allow one of their own, Gurrag, to teach shamanism in the Exodar. The Timbermaw, meanwhile, do not have the luxury that the Stillpine have of being isolated with potential allies on a small island chain, but rather must contend with various corrupted furbolg tribes in both Felwood and Winterspring. As a result, they have opened up relations with both the Alliance and Horde in an attempt to stem the tide of decay afflicting their people.
The corrupted tribes of Kalimdor include the Deadwood, Winterfall, Foulweald, Gnarlpine, Thistlefur and Bristlelimb. Not all furbolg tribes who are corrupted stay that way: The Blackwood furbolg of Darkshore, as an example, were aided by the nearby night elves who slew the satyrs working to corrupt them. In general, however, corrupted tribes seem to outnumber their uncorrupted kin, a harsh legacy of the Third War and the taint the demonic host spread across the land.
Ironically, while the furbolg of Northrend have managed to avoid this difficulty, it can't be said that they've avoided corruption itself. While the Burning Legion did not directly set foot on the slopes of Northrend, the touch of the foul entity Yogg-Saron has set the various tribes of the furbolg homeland against one another. Worse, in this case it is the furbolg themselves who created the opportunity for the old god of death to act. The furbolg city of Grizzlemaw, home to the Frostpaw tribe and set within the splintered trunk of the fallen world tree Vordrassil, has gone from being a strong center of furbolg culture and a shared home to many of Northrend's tribes to a battleground fought over by blood-crazed bear men unable to see reason.
As we know, during the War of the Ancients, both Ursoc and his brother Ursol were slain in battle with the horrific forces of the Legion. (Before Rhonin, Krasus and Broxigar went back in time, the furbolg themselves did not fight in this war, but after they interfered, the furbolg were counted among the loose allies of the night elves against the Legion ... Yay for temporal mechanics.) The bear gods were among the first of the ancients to answer the call of Cenarius and stand against the invaders, and they died defending ancient Kalimdor, struck down by monsters, but the furbolgs did not abandon their gods so easily. Their shaman came up with a plan to use the remaining power within Vordrassil to grow a magical fruit capable of raising the bear god from his state of death. Unaware of the reason their old neighbors the kaldorei had abandoned the tree (namely, that its growth had put down roots into the ancient prison of Yogg-Saron itself beneath Northrend), they went ahead with their plan and grew the mystical fruit. While it did raise Ursoc from the dead, it also allowed Yogg-Saron to taint him and, through him, all the furbolg of Grizzlemaw, pitting the Frostpaw and Redfang against each other.
Ironically it is the coming of a southern furbolg, Tur Ragepaw, that eventually sets in motion Ursoc's redemption. Interestingly enough, while most furbolgs are shamanistic, Tur is clearly a druid (able to shift forms between a healing tree, a bear and a moonkin) come north to help save Ursoc, who the southern furbolg also revere. Consulting with Ursoc's children Kodian and Orsonn, the furbolg druid and a band of adventurers (that's code for "possibly you, if you did the quest") managed to destroy the corrupted form of Ursoc and purge his spirit of the old god's essence. At present, this doesn't appear to have ended the hostilities at Grizzlemaw, as former allies tear one another apart.
The ultimate fate of Ursoc's brother Ursol, the battle between the corrupted and uncorrupted furbolg tribes of Kalimdor, and the pernicious madness inflicted upon the Northrend furbolg tribes by Yogg-Saron are all still waiting for a resolution. The furbolg have endured great tragedy through no real fault of their own, not having asked to be in the path of the Burning Legion (twice!) or to have Yogg-Saron interfere with their attempt to bring their god, their inspiration and guide, back from his undeserved death. Pushed to the brink, it's no wonder many of the furbolg are losing themselves to the savage impulses that are part of their legacy, turned against them.