Good add-on with reasonable price. I love it shows the browser icon for that particular message rather than showing the last used browser for all messages like the Country Flags add-on.
Glad you like the addon and found it satisfactory. The reason it shows the flag across all messages is to show where the user is at currently, not at the time of the post.
-The icon for 8.1 and 10 is the same. It should be different
-It says OS X, which should be macOS for newer systems
-Firefox version number could be outdated
-Chrome icon is not material design
-Lacks customization options