In the Network tab, when you edit and resend a POST or PUT request, if you modify the length of the request body, the Content-Length header value will still reflect the original unmodified body length. You can remove the Content-Length header before resending the request and it gets autotmagically replaced with the correct content-length upon sending.
However if you don’t remove the content-length header or change its value to the new body length before sending, the request just hangs. This can be super frustrating when you forget to delete the content-length header after manually crafting a large request and having to do it again.
It would be awesome if the content-length value was automatically removed or updated to the body length without having to remove it or modify it manually.
I agree, this is annoying and should be solved. I filed https://bugzilla.mozilla.org/show_bug.cgi?id=1623004 to track it and discuss further on which route we should take to make this work.
Thank you 
Just for posterity, this landed in Firefox 77 thanks to a contribution from ojaswa1942.
Looks like the problem described here is back.
Now the value of the Content-Length header not only remains unchanged, but now it also cannot be deleted at all, since there is no possibility to delete the Content-Length header on the new UI
The only ability to influence on Content-Length header that I have found is to re-declare it as additional header manually
Which in my opinion is even more inconvenient and annoying than the problem described here two years ago
This issue has been partially mentioned as part of another issue discussed in File form payload corruption (byte 0xD) when resending a request in the network tab
However, I think it is appropriate to consider this as a separate problem, who knows, it may be easier to fix it and take less time