FWIW, they’re not going to fix this in Gecko for the time being, because the relevant code is already a mess and long due for a complete overhaul. Currently, the entire rendering code is being reworked to use Cairo as the backend, which is supposed to allow taking advantage of hardware acceleration. It will also get rid of most of the 2D drawing code in Gecko, since that will be Cairo’s job. In the course of all that, the text layout code is due for a massive refactoring, and one of the byproducts of that will be soft hyphen support.
The bottom line is that it will take quite a while yet until Gecko has that support, but it will happen.
Makeshifts last the longest.