The setting adblockplus.please_kill_startup_performance
to "yes" as mentioned in the above post didn't help in my case either. This is due in part because in the event FF is using cache to load the initial startup page the load for the more recent versions of FF has become too fast for ABP.
Again this is because the issue is that FF, as is the case with any web page first started up, will use cache as long as the cache is saved (retained) which is for the purpose of speeding up the page load.I've apparently solved my specific issue by setting FF to clear cache on exiting the browser.
You do this by going to FF "Options
" > "Privacy
" tab > Click "Clear history when Firefox closes
" to check the box > Click on "Settings
" > Click on "Cache
" ONLY to check this option and click "OK
Note: If you haven't cleared cache for sometime it will likely take awhile to finish clearing it out which means FF will run for awhile after exiting hence you'll typically need to wait awhile before launching again as the instance of FF will remain during the time cache is cleared.
This web site Force Firefox to reload from the network on startup
provides and example how to do this, but BE SURE AND JUST CHECK THE "CACHE" SELECTION (NOT THE OTHERS as shown in the example)
as this is all that needs to be done for the purposes of this topic in the "Settings" option of the "Clear history when Firefox closes" option of the "Privacy" settings for FF.
Now after this change Firefox cannot use cache the first time it loads my startup page after initially launching the browser (cache will be used during the browser session however which is a good thing). An argument can be made that it's a good thing to clear cache now and then to save on memory glut anyway regardless of this specific issue of ABP occasionally failing to block adds due to the load speed of the more recent FF browser versions.
Let me know if this solves the problem as far as ABP occasionally not blocking adds in FF as well.