Latest version is worse Rated 2 out of 5 stars
This extension is potentially very useful. It has a nice looking status bar icon, however it installs in the locked portion of the status bar making it problematic to move (if you want to rearrange your icons).
The toolbar that gets added looks pleasant (it has a decent design) and it is not overly cluttered, but I don't like extra toolbars so I turn it off and just use the status bar icon to check whether I have mail.
Seeing as mail.com can collect mail from various other email accounts, this extension can potentially notify you of mail in pretty much any of your email accounts.
The downsides of this extension are that the status bar icon is a bit slow to log in whenever you open Firefox, and also the mail notification updates are slow too (i.e. you don't see the 'new mail' icon until a few minutes after the email has arrived). The same slow updates also apply after you've read your emails (i.e. the email icon doesn't reset for a few minutes, so for a while it still looks as if you have new unread emails, even though you don't).
However, the biggest problem is with the changes made in the latest version of this extension. I updated the extension, and without any warning or permission it just changed the page that is displayed when you close your last tab in Firefox. This overrides any settings that you have in your browser (e.g. Tab Mix Plus) and it forces the mail.com search page on you. It also ignores all the settings from your previous Mail.com mail check version, and just resets everything.
This kind of cavalier and arrogant attitude towards users really annoys me. I decide what pages to show in MY browser, NOT mail.com. There isn't even any setting to disable this irritating behaviour, so you're stuck with it permanently, unless you uninstall the extension.
I ended up reinstalling a backup of my Firefox profile so that I could go back to version 2.5 of Mail.com mail check (which doesn't change which pages your tabs show) and I have now blocked the mail.com mail check extension from updating so that I don't have to put up with the crappy new version over-riding my settings and forcing me to view pages that I don't want to see.