I posted about this issue here: https://support.mozilla.org/en-US/questions/1192412#answer-1045565
I have an addon that displays a badge with numbers in it. If the number has 5 or more digits, instead of displaying 4 digits only, the badge should display a ‘+’ (plus sign) indicating that there are more digits. Unfortunately, Firefox doesn’t support this. As posted in the support forum, using ‘-’ (minus sign) works, but using the plus sign doesn’t.
I think this is an issue with the font Firefox uses for setBadgeText. I’ve attached two images that show how it looks like with it set to display the plus sign and the minus sign.
EDIT: I’ve also added an image showing how it looks like in Chrome.
EDIT²: tested using Firefox 57.0.1