Hi vickilc, several users have reported problems with the Yahoo! toolbar getting disabled. I don't think we have tracked down the reason for this just yet. If I find a good answer, I will post back.

Meanwhile, could you check something? Sometimes Firefox does not successfully update a settings file named prefs.js and stores your changes to a temporary file. This only works during the same session, and when you exit and start Firefox again, you're back to the previous settings. This article has suggestions on how to check for that problem, which usually is indicated by extra files with names like prefs-1.js, prefs-2.js, and so on:

How to fix preferences that won't save

Followup:
The Yahoo toolbar add-on was updated on December 29, 2015. The latest version of the Yahoo toolbar fixes an issue that caused it to become disabled in Firefox 43.