I would not advise a downgrade to 8.0 as this version is no longer supported, or maintained, thus it could easily become unstable causing even further complications than you currently have. Visit & take note of the latest Firefox issues first.

See: Latest Firefox issues

Next, troubleshoot images & animation not showing

See: Fix problems that cause images to not show

I went through the 2 links, but it still doesn't display the thumbnails. I have noticed that yahoo mail has really slowed down in the last couple of days. I tried it ising chrome and some displayed and some didn't, so maybe it's a yahoo problem.