If I do a Google search then left click on a result, nothing happens. Right click and Open in new tab or Open in new window works okay.
In Seamoinkey Safe-mode it works correctly.
I disabled all my add-ons, no help.
Removed user.js, no help.
Removed prefs.js so a new, default prefs,js was created and it works correctly so something I did in prefs.js is causing this but I can't figure out what. The only thing I've been doing is in about:config, trying different UA strings to see if some websites are having sniffing problems.
I have added several site specific UA overrides for some sites like:
general.useragent.override.att.com;Mozilla/5.0 (Windows NT 6.1; WOW64; Trident/7.0; rv:11.0) like Gecko
general.useragent.override.att.net;Mozilla/5.0 (Windows NT 6.1; WOW64; Trident/7.0; rv:11.0) like Gecko
general.useragent.override.ebay.com;Mozilla/5.0 (Windows NT 6.1; WOW64; Trident/7.0; rv:11.0) like Gecko
general.useragent.override.google.com;Mozilla/5.0 (Windows NT 6.1; WOW64; Trident/7.0; rv:11.0) like Gecko
I can't think of any other settings I've changed in there around the time this started happening (2 weeks or so).