You can try to put the rule
-moz-border-radius-bottomleft: 0 !important; after the other two rules.
The reason you are facing the above warning is that a
order-alphabetical rule ignores a prefixes. If the two properties are only different in their prefix, then the order doesn't matter.
Few days ago the Brackets CSSLint supports the global preferences. You need to just open it with your Debug > Open Preferences File and simply add the rules which you want to be ignored like in the below example:
You can also find the rule names by opening up the csslint code and by searching for your warning you want to hide.
This way is much more ideal than just editing the extension code by your hand every time it updates.
Hope it helps you in fixing your issue.