No, you don't need H T11 to down Firelands bosses. However, you picked the wrong boss to start off with, as Beth'tilac very much tests your gear. If you are undergeared, it's harder killing all of the spiders in time, surviving the massive AOE, and healing through all of the damage. A much easier boss to start off with is Shannox. There is comparatively very little AOE damage, much easier DPS requirements, and focuses more on skillful execution than out gearing the various mechanics.

My guild only cleared T11 the week before the nerfs hit, so we didn't have ANY heroic gear, just decked out in pretty much full 359. Beth'tilac was hell on the healers, while Shannox was something we could do as it only required proper execution. After some time feeding off of Shannox's drops and gathering BOEs and rep gear, we finally downed Beth, and quickly did Baleroc and Rhyolith soon after. With the Firelands, you don't have to focus on one boss first because it is "conventional". My team did Baleroc (conventionally 5th) before Rhyolith (conventionally second or third), because the controlled placement and movement was a lot easier for us to accomplish rather than the crap on the screen explosion and RNG of Rhyolith.