Sorry for the late reply. I filed a bug for it this afternoon. I don't know of a real fix because it's really unclear what is causing it.

Summary of the Problem

When you do not use the Reading Pane, Outlook displays messages in front of the Inbox. There is some strange behavior with links and text that appear in front of text in the Inbox message list. It seems to start after the second or third message is opened (if you close between messages).

Areas of the message that are in front of sender name or subject line text in the Inbox do not react to the mouse; instead, it seems like the mouse is operating on the lower layer. So if you click in that area, Firefox will select the message under the mouse pointer in the Inbox instead of clicking text or a link in the message. It's really weird.

Workarounds

These are the ones I know of at the moment:

(1) To navigate the Inbox, open the first message you want to view, then use the up and down arrows above the message instead of closing and opening. Not always practical.

To use the scroll wheel, move the mouse pointer to the right edge of the message so it's not over any part of the message subject line.

(2) If you have a wide display, you could try using the Reading pane on the right side of the Inbox. However, if your display is not wide enough, there will be an inconvenient amount of horizontal scrolling, and the Inbox format changes so you see fewer lines of messages. (Example screenshot attached.)

(3) Switch to the Outlook beta layout by sliding the "Try the beta" switch that appears above the Inbox. You may like this better or prefer the older layout.

When the problem appears you can use Zoom to correct the mouse behavior (Ctrl+ or Ctrl+ or hold Ctrl and scroll the mouse wheel; on Mac, try Command instead of Ctrl). This is only good until you close the message and open the next one, so works best in conjunction with workaround #1.

Hi phyllys, if you don't like the Reading Pane or Beta, here's another workaround, a settings change:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

(2) In the search box above the list, type or paste stylo and pause while the list is filtered

(3) Double-click the layout.css.stylo-blocklist.enabled preference to switch the value from false to true

(4) Double-click the layout.css.stylo-blocklist.blocked_domains preference and set the value to live.com and click OK.

(In the unlikely event that you already had a site listed add ,live.com to the list.)

Then exit/quit Firefox and start it up again to make the change effective.