this is not my work
-Brooks Nelson
Brooks Nelson, Chemist at University of Florida
Answered Oct 12, 2018 ยท Author has 368 answers and 54.1k answer views
My limited understanding is you need pressure, temperature and enough elements that can fuse. If the temperature and pressure aren't high enough and/or you don't have enough elements that can fuse, then no fusion.
In fact I've never heard of fusion in a nebula, only in a star. The exception being a brown dwarf, which is considered substellar at 10 to 90 Jupiters in mass, and they can fuse deuterium (if over 13J) and also lithium (if over 60 J). But the burn through all of it in about 10 million years and wouldn't emit light like a main sequence star would.