Check your zoom controls.

<Control> + (plus) enlarge <Control> - (minus) reduce <Control> 0 (zero) restore

Check with the site support if there is still a problem.

If the problem messages have the characteristic fixed width "Courier"-style font, then here is my suspicion:

Firefox recently changed how it handles blocks of "preformatted" text. Previously, they were always wrapped to fit the window, but now, if the site does not set them to wrap, then you get a horizontal scroll bar. I can illustrate with a long line like the following:

This sentence is preformatted text which, on this forum, will not be wrapped but instead will have a horizontal scroll bar. 
(Which is no fun!)

If you right-click the problem text and choose Inspect Element (Q), Firefox will open the Inspector in the lower part of the tab and you may be able to see that the problem area is all within a <pre> tag.

Can you check and confirm that, or is something else going on?

As a quick check, you could try this:

If the Inspector is still open, click Console along its top edge. If it's not still open, press Ctrl+Shift+k to open the Web Console in the lower part of the tab. In the blank at the bottom next to >> type or paste the following long line of script and press Enter to run it (it injects a new style rule into the page to wrap preformatted elements):

var r="pre{white-space:pre-wrap !important}"; var s=document.createElement("style"); s.type="text/css"; s.appendChild(document.createTextNode(r)); document.body.appendChild(s);

Does the too-wide area now wrap?

There likely is a convenient way to inject that into Yahoo! Mail if it works, but I'll need to experiment a bit before making recommendations. Let me know how it goes.

By the way, some of your other symptoms sound as though they might be related to content-filtering add-ons. Could you test in Firefox's Safe Mode? That's a standard diagnostic tool to deactivate extensions and some advanced features of Firefox. More info: Diagnose Firefox issues using Troubleshoot Mode.

If Firefox is not running: Hold down the Shift key when starting Firefox.

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

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

 other than Presentation.
Text in console does not help.

Inspector does not show any Pre other than Presentation. Text in console does not help.

Zoom Control - does the trick. Was wondering where that option might be.

Safe Mode did not help. It's as if all panes except Ads have been resized - Image now included.

Hmm, was the screen shot from after you reset the zoom level or before?

The center column seems a bit oversized compared with the others, but I don't have a good guess as to why.

I don't know with that type of ad;

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jscher2000 - The image was with no zooming. Zooming down will get rid of the blank space and show the Mail Search and Tools Gear, but only for that email. Zoom down 2ce will get the bar with Mail, News, Sports, etc. Soon as I go to another message the Mail Search and Tools Gear are again hidden.