FF mozilla is a Browser not a email provider. Your question is misdirected here.

It comes up on other browsers when I use Yahoo Mail. The calendar is a button on the mail page. I thought there might be some quirk with Mozilla where the calendar won't work.

Hi betrayed, I have a test account for Yahoo, and I have just tried opening the calendar from Yahoo Mail using Firefox 61 on Windows 10. It worked for me.

Let's try something simple, like clearing your cache and cookies.

1. Click the Library button and choose History.
2. In the History menu, choose Clear Recent History.
3. In the Time Range to clear drop-down, select Everything.
4. Click the arrow next to Details to display the list of items that can be cleared.
5. Select both Cookies and Cache.
6. Click Clear Now.

In addition, try updating to the latest version. For instructions, check out the article: Update Firefox to the latest release

If the problem still occurs, we'll need some more info about your Firefox setup. Go to [=] > Help > Troubleshooting Information then click Copy text to Clipboard. Open a reply to this post, and go to Edit > Paste to paste the info from your Troubleshooting Information page.

Let us know how it goes. :)

Are you able to load the page using its address instead of the button?

https://calendar.yahoo.com/

That page actually is much simpler than the mailbox, for one thing, it doesn't seem to have advertising on it...

If a site is generally known to work in Firefox, these are general suggestions to try when it stops working normally:

Double-check content blockers: Firefox's Tracking Protection feature and extensions that counter ads and tracking may break websites that weren't built to operate normally without the blocked components.

Do you see a shield icon toward the left end of the address bar, near the lock icon? More info on managing the Tracking Protection feature in this article: What happened to Tracking Protection?.

Extensions such as Adblock Plus, Blur, Disconnect, Ghostery, NoScript, Privacy Badger, uBlock Origin or uMatrix should provide toolbar buttons 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 may need to click the button to see what's going on and test whether you need to make an exception for this site.

Cache and Cookies: When you have a problem with one particular site, a good "first thing to try" is clearing your Firefox cache and deleting your saved cookies for the site.

(1) Clear Firefox's Cache

See: How to clear the Firefox cache

If you have a large hard drive, this might take a few minutes.

(2) Remove the site's cookies (save any pending work first). While viewing a page on the site, try either:

• right-click (on Mac Ctrl+click) a blank area of the page and choose View Page Info > Security > "Clear Cookies and Site Data"
• (menu bar) Tools > Page Info > Security > "Clear Cookies and Site Data"
• click the padlock or "i" icon in the address bar, then the ">" button, then More Information, and finally the "Clear Cookies and Site Data" button

In the dialog that opens, you will see one or more matches to the current address so you can remove the site's cookies individually without affecting other sites.

Then try reloading the page. Does that help?

Testing in Firefox's Safe Mode: In its Safe Mode, Firefox temporarily deactivates extensions, hardware acceleration, and some other advanced features to help you assess whether these are causing the problem.

If Firefox is not running: Hold down the Shift key when starting Firefox. (On Mac, hold down the option/alt key instead of the Shift key.)

If Firefox is running: You can restart Firefox in Safe Mode using either:

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

and OK the restart.

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

Any improvement?