Was it an alert on an "infobar" about a pop-up window? You can edit that setting in any of these places:

• right-click a page on the site > View Page Info > Permissions tab
• while viewing the site: Tools menu > Page Info > Permissions tab
• orange Firefox button (or Tools menu) > Options > Content > "Exceptions" button to the right of Block pop-up windows
• type about:permissions in the address bar and press Enter

Does that work?

When I right-click a page on the site>View Page Info>Permissions tab, I can un-check the Use Default box for Open Popup Windows and select Allow vesus Disallow.

This enables me to sign into a page that will load in the same tab, for example. Previously, I would get a message saying Firefox prevented a popup from loading a page in a new tab or window and I could either a) block that from happening b) access my popup settings or c) never show that message again.

At the bottom of the window would be the URL address(es) of the popup page(s), so that if I wanted to access a page, I could simply click on the address and a new tab would open with the new page.

Since I clicked option c) never show this message again, I have to center click my mouse on the link to get the new page to open in a new tab.

Needless-to-say, if the popup blocker would just let the page load in the new tab, that would be the best of all worlds (the way it used to work, before my popup blocker went bad).

But, this is what I am left with trying to work around the problem: no new pages loaded in a new tab unless I center click the link and no popup blocker message with the options listed above.

To restore the original behavior, you could try removing the explicit site-specific permission by deleting the site from this dialog:

orange Firefox button (or Tools menu) > Options > Content > "Exceptions" button to the right of Block pop-up windows

Does that work?

Also, I don't recognize this part:

At the bottom of the window would be the URL address(es) of the popup page(s), so that if I wanted to access a page, I could simply click on the address and a new tab would open with the new page.

Oh, wait a minute, I just reviewed your "More system details" list again and it appears that you have a custom pop-up handler:

• Extension: Adblock Plus Pop-up Addon 0.7
• Modified setting: privacy.popups.showBrowserMessage: False

So if the above reset doesn't resolve it, you might need to review the add-on's settings in case it has its own list.

My bad-- it was the Adblock Plus Pop-up Addon 0.7.

I simply went to the orange Firefox button>Add-ons>Adblock Plus Pop-up Addon 0.7>Options>My site rules>right clicked the site>edit>checked "Block all popups" and unchecked "This rule applies to: "New windows" and "New tabs."

Presto! Firefox operates as before.

I didn't realize that I had controls within the addon that would affect Firefox's popup blocking action.

Thanks for calling attention to the "More system details."