I'm already well familiar with these tools and this advice, but I nonetheless appreciate you taking the time; perhaps someone else will benefit from their presence here.

Extensions aren't the problem, and Firefox 51 with e10s enabled is amazingly snappy by comparison with the 45 ESR that I usually run.

On the other hand, 51 on Windows also refuses to load or render content from files located in directories mounted via the admin shares (e.g. \\127.0.0.1\C$). Since I use this functionality very heavily (home directory mounted at H:, etc.), I can't really switch over until I've had the leisure to find and resolve the cause of the issue. I have to imagine it's some new security setting that I can modify in about:config, but no one else seems to have run into the same problem, and identifying the changes in config options between Firefox versions is not something I've yet found a way to make trivial.