Unfortunately all the workarounds range from "buggy" to "horrible". The obvious patch to Flashblock (apparently just setting a "yes, this plugin really is allowed to rewrite HTML" flag) doesn't work on some sites, Flashstopper only deals with flash videos and not flash in general, and Firefox's native click-to-play is clunky and too coarse-grained (no I don't want to enable all flash applets on a site just to watch a video).
Now in theory there would have been some warning when updating Firefox that the addon was incompatible, as each addon is supposed to declare which versions it works with. Except Firefox has followed Google Chrome's lead in continually releasing new major versions (presumably to be able to say "my version number is bigger than yours!"), with the result that the version number has no semantic meaning and version N+1 is equally likely to be fully compatible as it is to break everything. This means there's no point in specifying a maximum version that your addon is compatible with unless you want to have to release a new one every 6 weeks.
For added annoyance, the update appears to have reset the configured search engine back to Google (which is odd as Mozilla has just ended their partnership with Google). It also appears to be rather unstable - it's crashed a good half-dozen times already. I wonder if I'm better off just reverting back to version 33, or possibly going back further to version 31 (which at least will continue to get security updates, unlike version 33 which is now considered end-of-life despite having only been released for less than two months).