Firestorm may have a “local mesh” preview option, but developer Beq Janus notes it ain’t easy.

“I have a TODO item on my personal project list for build tool improvements, to add local mesh support to Firestorm (and ultimately contributing back to the lab so everyone gets it). It is, however, a lot of work.”

Via SL Community Forum: https://community.secondlife.com/forums/topic/416679-local-mesh-capability-in-some-viewer/