That is not something Firefox can do. It's a website option via the program of the web page.
Is there something you wanted to do?

Thanks for replying so quickly Fred.

In a spreadsheet i can grab a horizontal bar from the top of the right hand scrollbar, and drag it down to create a horizontally split view of the spreadsheet, and each pane can scroll separately over the same data.

I would like to be able to do this with any webpage, so that the top pane in the browser tab is scrollable over the webpage, independently of the lower pane, please?

As I said, this is a website setting. Not the browser.

One thing you can try is to open that web page in two different windows.

>As I said, this is a website setting. Not the browser.

yes , i realise that a header or banner would be set by the website, but i hoped a split window might be possible using a firefox addon. there was one but it doesnt work since about FF57

>One thing you can try is to open that web page in two different windows.

no, i tried that and unfortunately, it is very cumbersome, as i need to use the function frequently on many different webpages over long periods.

thanks anyway.

As you know, some sites have fixed headers that remain in place as you scroll down the page, for example, in this thread. This is accomplishing using the style rule position: fixed in combination with additional rules to ensure appropriate spacing for other items.

Most users have asked to get rid of fixed position elements, but it sounds as though you would like a way to apply this style rule to a certain block of content and then, perhaps, remove it when you are finished with that chunk.

The challenge is that web pages use a very diverse range of layouts, so creating a "push pin" tool to fix part of it to the top of the viewport (the rectangular area where Firefox displays web pages) would be difficult to automate. If there is a particular site where you have this need more than others, it could be a good one to experiment with. (Especially if volunteers can view the contents without needing a login.)

@ jscher2000 thank you. yes, i thought changing the layouts on different websites might be tricky. thats why i hoped it might be possible to split the viewport into two panes using an addon. there was one but it doesnt work since about FF57 because of a big change in Firefox.

however, this seems to be one of those problems that need a sledgehammer to crack a walnut. never mind. thank you anyway. regards H