Were you able to switch to any of those tabs and view the pages?

There are some travel sites like SeatGuru that open a pop-under tab or window when you click a link on the site. Maybe Orbitz does that, too. Firefox's popup blocker allows popups in direct response to a click because ignoring those would break too many legitimate sites. But windows/tabs should not open on their own without a valid triggering event.

Currently, while users can create and collapse or close a tab group, I don't think there is a way to hide an individual tab without using an add-on. Could one of your add-ons be implicated in hiding those five tabs?

I could not find a way to access any of those tabs. I closed ALL visible tabs but the hidden tabs were still open in background and one was consuming 2-3%... of what I don't know, clearly not CPU because TaskManager showed all of Firefox at about 1%.

Another place to check for non-visible tabs/windows would be Firefox View. Either:

• click the file drawer button at the left end of the tab bar
• type or paste about:firefoxview in the address bar and press Enter to load it

In the left column, click Open Tabs. Are any of the ad tabs in a non-visible window listed here?

So those background queries are likely not "hidden tabs" but rather domains for trackers/fingerprinting, cookies, and other ad related or embedded media related domains. Most websites have other domains that are queried as background processes and which appear in the task manager as if they were sites you are accessing. Check out a browser extension like disconnect to give you a list of all the tracking and background process domains that are being accessed on each website you access, it gives you a handy list or mindmap graph to visualize all the background process domains that are accessed for each individual webpage youre on and blocks domains that are known to be for ads and trackers

My question still stands. Firefox bills itself as protecting your privacy and blocking trackers... so why were there five trackers running in background?

Next time you see them, could you screen capture more details about those processes?