This addon is not even open source. Author can see all your email and contents in it. Look at his homepage and privacy policy. At least he is being honest about it.
It is upto you, users to trust this random man on the Internet with your gmail contents. I don't know why Mozilla has this under "Recommended" section.

"Checker Plus for Gmail" has gone from excellent to completely useless overnight! This extension worked well until a few weeks ago when the wheels came off:

I can no longer add gmail accounts! I have 3 accounts. There is a very weird "option" in the "Preferences" tab: "AUTO DETECT" or "ADD ACCOUNTS". I never had trouble with this in the past, but now if I select "AUTO DETECT" I cannot add an account, and if I select "ADD ACCOUNTS" I get an error message from "Checker Plus" when I try to add!

No idea what's causing this, but "Checker Plus" has now been relegated to the ranks of "NOT READY FOR PRIME TIME".

UI goes so ugly with the latest update

Themes

The UI changed slightly in the last update and I don't see an option to roll back to the previous version. I have OCD and autism and this drives me insane.

does what it says on the tin. it notifies you when you have unread emails. HOWEVER, recently there was an update that locked MANY previously free features behind a $20 USD paywall (despite the fact that the popup CLAIMS you can donate ANY amount, i guess $19 and lower doesn't count as any).
These features aren't anything advanced either, any and all customization, changing what buttons appear on the popup, the works. Way shittier add-on. Use only as a notification bell that opens gmail and nothing else.

This is a good extension! I would definitely recommend it, but I have to complain. Why are you expecting us to donate just to get extra features that shouldn't even be put behind a paywall? I know you want people to donate, but don't put simple things such as themes and being able show or hide specific buttons behind a paywall, it's just not worth it. I would only put the more advanced features behind the paywall. (Also I'm rating this one star since it seems like the dev doesn't want to reply to anyone.)

This addon cannot count the unread emails in my gmail account, always display the wrong number.

Hiding functionality behind paywall.
New update completely broke the app.

Used to be great, basically doesn't work anymore for anything besides viewing mail. Constantly requires you to attempt to log back in, and just throws a "something went wrong error".

None of the troubleshooting steps including completely different operating systems on different hardware solve the issue, it's just fundamentally broken.

after last update (V25.0.8.1) lost connection with account and can not setup again, because: "security_token not matched!"

no support, no nothing...

half features are behind a paywall

I have exactly the same problem as Jack. I don't understand why an author tinker with something that works...

I hit E on my keyboard on accident, and several of my emails are just gone. Not in trash, not in unread. No pop ups saying what happened to them. E is Archive according to the keybinds, and there are no option to change any keybinds or disable keybinds except of course the ever important option to change the R key from "set to read" or "reply", because why would you need anything else?

Now I have to sit here and use Shadowplay to find which emails were archived, and they all were several days old.w

So, after installing this extension, a page opens with a link "configure extension". But if you thought that pressing this link brings you to where you can actually handle the add-on's configuration, you're sadly mistaken. Because it really brings you to a page where the author is advertising himself and all of his other extensions.

I'm not yet sure how well this add-on works, but for the misslabeling of buttons and the blatant spam attempt, it starts at a disadvantage where the rating is concerned! If only I could apply a negative rating...

I love this app, but it quit working and gives me this: Error: trans abort: AbortError: A request was aborted, for example through a call to IDBTransaction.abort. (js/common.js 4544)

Deleted all my recent emails in multiple accounts.

More and more bug and bad behavior everytime Firefox update !! And Jason Savard is so Ridiculous, all theme feature are not free !!!! You have to pay for a dark theme or to use smaller font and icon !!! LOL WHAT A SHAME

Disappointing...had to remove.