did you change the JWildfire.cfg, for example to change the memory settings? When they are too high, the JVM can't start.
As it happens, I hadn't tried to use the Steam version of JWildfire since early January, and so it had updated and there is now a cfg with (so far as I know) just the default settings, including -Xmx8G which is lower than my default for the previous version I used (6.20, where I had it set to -Xmx14G).
I've updated my original post to include the text of the entire current JWildfire.cfg in light blue.
Many thanks for helping me with this!
— Sage Ealy-Silk