Firefox 43 shows additional white space in my extension

Add-ons
#1

I developed an extension for Firefox and
Chrome. the same extension works fine in Firefox developer edition and
chrome, but it show additional blank white space in firefox 43. my body
and html have 300px width, but my extension is bigger with a lot of
blank

Screenshot (20).png564×595 136 KB

#2

Can you show us how it looks on Chrome? Also, is the code available somewhere?