Hi e424932, I've seen five similar reports today so far. 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?

So when I disabled Adblock Plus in Firefox for mail.yahoo.com, I was able to view my contacts again. What bothers me is that when I access Yahoo Mail with Adblock Plus enabled on other browsers (Edge & Chrome in particular), I can still access my contacts with ads blocked but on Firefox, I must endure ads by disabling Adblock Plus in order to view my contacts. Frustrating. Perhaps I should contact the Adblock Plus developers to see if they are aware of this issue. Thanks for your help.

I just disabled AdblockPlus and reenabled it and the contacts returned!

What bothers me is that when I access Yahoo Mail with Adblock Plus enabled on other browsers (Edge & Chrome in particular), I can still access my contacts with ads blocked but on Firefox, I must endure ads by disabling Adblock Plus in order to view my contacts.

You could check and see whether the one in Firefox needs an update.

Yep same here! Disabled Adblock an contacts returned. Hopefully they can get this fixed.

I disabled AdBlock, closed firefox, re-opened it, opened yahoo! mail and still no contacts

I've found on another forum to turn of UBlock Origin and that has worked for me.

I disabled AdBlock, closed firefox, re-opened it, opened yahoo! mail and still no contacts

I've found on another forum to turn of UBlock Origin and that has worked for me.

There is a lot of overlap in what these two extensions do, so you might not need both of them running at the same time.

I also disabled AdBlock on Yahoo Mail but still can't see my Contacts and Calendar in Firefox (version 80.0.1, 64-bit). However, if I use the Chrome or Edge browsers, Contacts and Calendar both appear--even with AdBlock on. (I don't have, let alone use, UBlock Origin.)

Hi willlad-f, could you test in Firefox's Safe Mode? In its Safe Mode, Firefox temporarily deactivates extensions, hardware acceleration, any userChrome.css/userContent.css files, and some other advanced features to help you assess whether these are causing the problem.

You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" Help button > Restart with Add-ons Disabled
• (menu bar) Help menu > Restart with Add-ons Disabled

and OK the restart.

A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

Any improvement?

I solved this problem by disabling Privacy Badger for my ATT/Yahoo web page.