The flash errors definitely seem to be getting worse not better.
For me, Firefox in general has starting getting too clunky with recent releases. It seems to be going more towards IE and less like the real Mozilla (NN)
On my main machine i have kept a copy of Firefox version 11.0 which i use for personal browsing
i use the others (including 17) for testing purposes only
one thing i have noticed, renewing your profile every now and then tends to speed things up
(save bookmarks etc then add new profile)