Why in firefox web extension POST request call on content script but execute on background?

I have web extension working good in Google Chrome.

Among other things its send POST request from some page content script, get data from response and operate with data. It is necessary to send request from content script because Referer header need to get data in response.

When I try in Firefox last version this works some weird way: my POST request log located on background page Network Monitor (expected it should be on page when I have my content script ) and of course my POST request headers don’t contain Referer header so I can’t get data from response.

It’s no difference to use ajax or simple xrh - works same way.

What could be the problem?

The solution is this