IE or Edge debugger: this is what occurs if you removed SYSTEM account rights on current user folders because this service is run by SYSTEM account.
In that case, give NT AUTHORITY\SYSTEM account full rights on this folder:
C:\Users\< user account >\AppData\LocalLow\Temp\Microsoft\F12
This feature is now available in beta version (v67.x). You will need to go to about:config and enable the flag devtools.netmonitor.features.resizeColumns.
For previous versions see Change columns size in Firefox Developer Tools