As many other users mentioned, this stopped working the last time Capital One put out a new version about a month ago.
For me, when I click on the Eno logo and "Checking Session" appears, if I need to sign in a new window pops up for the login but then immediately shrinks and closes with out giving me the opportunity to put in my credentials. I was able to make it work again for 2 uses by uninstalling and reinstalling the add-on. Also, the work arounds others mention, go sign into C.Ones website, then open a new tab (or three) on the same retailer allow me to struggle through.
Honestly, I may drop Capital One if this isn't fixed, there are competitors with apps that work.

PLEASE MAKE IT WORK WITH FIREFOX!

The app used to work fine. However, as of 10/25/21 it just pops open a window showing my existing virtual cards but doesn't allow me to create a new one.

virtual card will not add new maechants

Used to work, now broken as other reviewers have noted.

I have had the same problem so well described by other reviewers for about a month. It used to be a great add-on! "This addon has effectively been nothing more than a way to log in to the Virtual Cards page of the Capital One site for MANY versions now. It does not create new Virtual Cards and does not allow you to lock or unlock existing Virtual Cards. This is all but useless for anything other than getting you to the Capital One page that allows you to see your existing Virtual Cards. This addon has been the one and only way to create new Virtual Cards since their inception (you can't do that on the website), but now even this addon doesn't do that." When Eno stopped working a year or so ago, Capital One blamed Mozilla and I am sure Mozilla blamed Capital One. I wish they could work together to fix the current issue.
I tried the solution mentioned by Crix, but it didn't work. Not sure what adblock is or how to disable it.

when it works it works, but then it just breaks for no reason and you can utilize it. They just need to have this functionality on their own website or app, because this is annoying

This addon has effectively been nothing more than a way to log in to the Virtual Cards page of the Capital One site for MANY versions now. It does not create new Virtual Cards and does not allow you to lock or unlock existing Virtual Cards. This is all but useless for anything other than getting you to the Capital One page that allows you to see your existing Virtual Cards. This addon has been the one and only way to create new Virtual Cards since their inception (you can't do that on the website), but now even this addon doesn't do that.

Does not work with Firefox. It used to be wonderful before it was changed. Will use another credit card now for virtual card numbers. Capital One was better than Citi Bank for virtual card numbers but I will go back to Citi Bank now if compatible with Firefox.

Sort of worked, then doesn't, won't autofill, very frusterating. If it worked it would be great. It doesn't.

This add on doesn't work anymore. Many months ago it stopped remembering your login details.

Now in the last month or so, the popup login window doesn't even log you into the eno window. It just opens your virtual cards. If you want to create a new virtual card, you can't. Whatever they did to change it, they ruined it.

This WAS one of the most useful add-ons I have in Firefox. However, currently it is utterly broken. When you attempt to log in, rather than launching the app to generate or use a virtual card, the login takes you to the card management webpage, while the app waits for the login.

Failed to work, so uninstalled it after multiple tries - expected better from CapitalOne since have great service otherwise :-(

This used to work great. Now it hangs up or takes me to existing virtual numbers, not allowing generation for a new number. So far this happens only on Firefox (Mozilla). However, it works on Chrome, at least, so far. I try to use Firefox as it is supposed to protect privacy, while Chrome is supposedly a spy machine, but I have been told by Capital One IT people (more than one) that Eno is more compatible with Chrome; although, they were careful not to promote it over Firefox; they were simply stating as a fact.

used to work perfectly, now it will not work

Like everyone else, this used to work and now it pops up a login window to only manage your virtual cards, the little eno window just spins waiting for login. since everyone is experiencing this issue, very disappointing they obviously did no testing of add-on for firefox. Using latest 3.6.1 version.

Fantastic and much needed idea and feature!!

However, the implementation of the idea is just awful. The add-on either:

works only sometimes,
not at all
or takes 15 minutes to load.

AND everyone providing a review are all sayin the same exact thing!!!

You would think that Capital One would want to fix that or at least "take it very seriously" I just end up using another service that actually works as designed.

7/2022: once again it has stopped working
7/2023: It worked for a while, but now it is broken again.

Basically is worthless when you need it and Capital One doesn't seem to want to fix it. It'll pop up and connect immediately on pages you don't need it but on pages where you need it to operate it'll freeze up trying to connect. Even worse, if you try to disconnect the session it'll still keep trying to connect in the background until you close out firefox in the task manager. So not really worth using until they pull their heads out of the "exit hole" and fix it unless you got a lot of ram to waste as it'll keep eating up resources and not work when needed.

it does not work makes no sense

Often hangs and is unable to proceed without going back into the shopping cart. I lost an order I had alerts setup for when it got in stock. Because of the app issue it went out of stock before I could fix the issues and get the virtual number. Please make this app more robust. I pay yearly for the card but this app is not there when I need it.

Did not work at all. added to mozilla firefox. Said to click to set up. I clicked. Wait. Wait. Wait. Shows Capital One/Eno small browser frame on screen saying Continuing. Come back hour later. Still says that on the screen. Deleted and reported to Capital One that is does not work.

Updated July 2022,
Um, an old problem is back, guys. It gets into a Sign In through Browser --> Login popup --> Sign In through Browser.

Based on other reviews, it has been broken on Firefox for three days. The devs released a broken product and they're not fixing it.

Would give it a negative star rating, because I've given better products 1 star.