Do you mean flashblock doesn't work at all, which was true a few weeks ago, or did you mean there are some bugs using it with certain sites?
If not working at all, did you update to the latest version 1.5.18 update from a few weeks ago?
https://addons.mozilla.org/en-US/firefox/addon/flashblock/
I'm using it on Mavericks, but there was a major problem using the previous flashblock, which appeared to be due to some changes introduced in Firefox 34.0, nothing to do with Yosemite. Now fixed in the latest 1.5.18 update.