Is the problem only in Firefox, or other programs also?

Update / re-instal the mouse program.

Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac Options) key, and then starting Firefox. Is the problem still there?

FredMcD said

Is the problem only in Firefox, or other programs also? Update / re-instal the mouse program.

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Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac Options) key, and then starting Firefox. Is the problem still there?

No problems with the MMB on other programs, just Firefox. After I made the changes yesterday (and right after posting this help question), when I clicked with the MMB I would get a last closed page tab instead of the URL I clicked. I changed duplicatethistab.bfhistory.middleclick.open_in_duplicate_tab; back to false and now when I MMC I get a duplicate tab instead of the URL I clicked on. I'm going to change duplicatethistab.link.middleclick.open_in_duplicate_tab; back to false and see if that cures that problem. As far as reinstalling the mouse program goes, I never installed a mouse program from the start. Ubuntu installed the mouse for me. I checked for any mouse drivers for my Logitech M510 wireless, but they didn't have any for Linux.

I made the last change I mentioned and that seemed to do the trick, a MMC will now open the URL in a new tab like I wanted. Apparently this is an intermittent problem, sometimes it will accept a MMC and other times it won't. I haven't installed any new add-ons or plugins, of course every time Firefox updates, within a few days add-ons and plugins will update too.

Keep an eye on the problem.

Note that duplicatethistab prefs used by the Duplicate This Tab extension, so it is possible that this extension is causing the issue. Note that this extension is no longer maintained by its developer.

That may be the problem. After I posted my last update my RMC started working like it's supposed to up until I opened my 5th or 6th tab, then as usual it stopped functioning. I'll disable Duplicate This Tab and do some surfing to see if that takes care of the problem. I'll get back to you on whether this works or not. Thanks for all the help, you guys are Rock-Stars!  ;)

I disabled Duplicate This Tab as suggested, took it for a spin, and so far it is working as it's supposed to. I've been MMCing on just about every link I could find on different websites and it has worked great. Though there are times on search engine pages like DuckDuckGO, that I have to hold down the MMB a little longer and/or on different spots on the link to get it to open the tab in a new tab. Hey, that's acceptable to me, at least it works now.  :)

 I forgot that Duplicate This Tab was an add-on, I used it on a daily basis and will miss that function.  Perhaps one day someone else will come out with a like add-on.
 Anyhow, I really appreciate all your help fella's.  You are another reason why I love Firefox so much.  Hopefully I'll never have to bother you guys again.  ;)

Contact their support and ask questions.

Let us know if there is a fix

FredMcD said

Contact their support and ask questions. Let us know if there is a fix

 Which support are you referring?

 I thought I had marked this as being solved, and I don't see anything on this page that shows it as resolved....

The post by cor-el is marked as solved or were you thinking of something else?

Fred was saying to contacting the Duplicate This Tab extension author. Though as cor-el pointed out this extension is no longer maintained anymore as last update was July 19, 2012 and noted in About this Add-on details.

https://addons.mozilla.org/en-US/firefox/addon/duplicate-this-tab/

Understood. Thanks.

I uninstalled Duplicate This Tab and installed Tab Mix Plus, which blows the doors off of all the other tab add-ons. My MMC's respond even faster now, and there's a ton of other bells and whistles. I wish I knew about this a long time ago...  :)

 Thanks guys for all your help....