You only get a tab highlight (glow) if the URL changes as a consequence of the reload (titlechanged).
See line 1498: chrome://browser/skin/browser.css

Such a Tab should get an unread="true" attribute if the content is refreshed, so maybe you can use that instead.

Add code to userChrome.css below the default @namespace line.

• http://kb.mozillazine.org/userChrome.css

The customization files userChrome.css (interface) and userContent.css (websites) are located in the chrome folder in the user profile folder.

• http://kb.mozillazine.org/Editing_configuration

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* only needed once */

.tabbrowser-tab[pinned][unread] {
 background-image: -moz-radial-gradient(center 3px, circle cover, rgba(233,242,252,1) 3%, rgba(172,206,255,.75) 40%, rgba(87,151,201,.5) 80%, rgba(87,151,201,0))!important;
}

Unfortunately that doesn't solve my problem. If I set up my userChrome.css with your code, I get the highlight every time the app tab is completely refreshed (by opening a closed FF, or through ReloadEvery addon) but not when I get a new email. Whatever mechanism owa uses to update the inbox while I'm off in another tab does not trigger the highlight. What you said makes sense though, gmail changes the title to Gmail Inbox - (#) whenever you get a new message. Any other ideas?

I found a way of doing exactly what I wanted by redoing an OWA refresh and keep alive script I found (link). The script I set up is here: http://pastebin.com/T1Yffywm