In the Tab Mix Plus options, click on the "Events" button, "Tab Opening" tab, and uncheck the options you want under the "Open tabs from" section.
Helpmeh did not have tabmixplus installed and therefore the Options could not have possibly been set to have those boxes checked nor could tabmixplus have been enforcing them. I can see the logic but consider the following, which is how I'm envisioning this and why I think we're going about this wrong, with addons and so forth:
Any addons settings only exist if you have the addon installed or had it installed and it didn't clean them up. This makes sense.
The options an addon sets are options relating to the function of that add-on. Firefox, without the addon, will not heed those settings because it is unaware of them.
With that in mind, we must consider that the options suggested are already unchecked, since helpmeh never had tabmixplus installed, and additionally that even if it was somehow checked without tabmixplus installed to enforce it then firefox uses it's default behaviour.
In this case it is Firefox's default behaviour that seems affected, or perhaps, another add-on.
My suggestion would be to Shut down firefox, reboot the PC, and see if that helps (assuming of course you actually hadn't rebooted it since starting this thread) I had a similar but slightly different issue whereby all links opened in new tabs no matter what I did. quitting and restarting firefox didn't fix it, but rebooting did.
Also, you might try starting firefox in safe mode, with no add-ons, and see if the problem persists. If it does not, then we know the issue is that you have an addon causing the problem, not that some miracle add-on will cure it.
Also sorry if I seem a tad "short" here, but if there is one thing that I know doesn't solve a software problem is piling more software on top of it.
edit: Tabmixplus is a great addon. Can't say I find the default options appealing (red italic text?) but it's customizable and has a lot of options