Like others I tend to accumulate a lot of Tabs as I browse and I find this Add-On to be very useful in helping me know just how many are open.
It's also reassuring to know that it doesn't require any permissions.
I read earlier that the Tab Count font size could not be increased due to Firefox's developer restrictions and so I have chosen to have red text on a white background for increased readability as was suggested by the developer.
I like the fact that the counter updates in real time eg. when I issue a command to Close tabs on the right, I can keep an eye the count as it decreases (my tab count can be quite high).
In these days of increased RAM and increased Tabs, this extension genuinely fills a gap.