Should i file a bug report ?
55 vs 54 sidebar render comparison screenshot -
https://drive.google.com/file/d/0B6yP6TaDp4R_RjJSZi1hZDNwR3c/view?usp=sharing
In your "sidebar_action" entry in the manifest, you are not opting out of "browser_style" which means Firefox will add a style sheet to your sidebar. That sheet probably changed.
“browser_style” fixed the problem,
Thank you