CentOS 8.x Firefox See HTTP headers in Firefox

To see HTTP headers in firefox use following steps:

Go to menu on top right corner -> Web Developer (or Developer) -> Inspector (Shortcut key is Ctrl+Shift+C)
Among various tabs that open below click on Network.
Then among various GET requests made for that page (css, js, png, etc.) select one for which headers need to be seen
When you select a particular request both request and response headers are visible on the right side
There is also option to edit the request headers and send a new request with updated headers.

Refer:

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