Live HTTP Headers is a famous add-on for Firefox allowing users to modify and replay HTTP requests.

However, The replay button after Firefox latest updates is not working . here is how to fix it : 

A- Close FireFox browser .
B- Clik Start Menu, Choose Run. (Keyboard Shortcut : Win + R)
C- Type " %userprofile%/appdata " , Hit Enter .
D- Search For " livehttpheaders " .
E- Right click and choose to open by winrar .
F- From winrar window, open " content " Folder, Then double click " LiveHTTPReplay.js " . ( Suggestion: Notepad++
G- Navigate to line 32, at the end of the line replace the " ; " with (+ "If-Modified-Since: *\n";) .
H- Save the file, Winrar will ask you to update the archive as the file was modified, Choose yes .  

Now Replay button should works fine . 

Youtube Tutorial :