Hi jetjack, is this problem on Yahoo Mail? I've seen five other reports of this today. One user reported that their ad blocker was hiding the contacts, perhaps due to a layout change by Yahoo. Could you:

Double-check Content Blockers

Firefox's Tracking Protection feature, and extensions that counter ads and tracking, may break websites that embed third party content (meaning, from a secondary server).

(A) The shield icon toward the left end of the address bar usually turns a bit purplish when content is blocked. Click the icon to learn more or make an exception. See: Enhanced Tracking Protection in Firefox for desktop.

(B) Extensions such as Adblock Plus, Blur, Disconnect, DuckDuckGo Privacy Essentials, Ghostery, NoScript, Privacy Badger, uBlock Origin or uMatrix usually provide a toolbar button to manage blocked content in a page. There may or may not be a number on the icon indicating the number of blocked items; you sometimes need to click the button to see what's going on and test whether you need to make an exception for this site.

Was the problem caused by one of those?

On September 2nd Firefox did a large update. It wasn't until the next evening that I noticed my contacts were gone and then I found out that all my bookmarks were missing as well. After reading through many ideas on youtube and Firefox, I did a refresh. That seemed to have restored my contacts. The bookmarks may have been there all along but I had to play around with so many things that I do not remember how I got them back. Now I also just noticed that my Add Blocker Plus is not present. I may have to reload that. I do not give up easily when the greater powers decide that they can make our browser or what ever run better, only to mess things up for a lot of us. Thanks for your response.