When you pin a tab, in Firefox , you should not be able to change the url. Any links that navigate away from this tab, should open in a new tab (in the same container [if applicable]). It kind of defeats the purpose of having a pinned page/site.
The navigation bar should be grey out and if you hover over or click the navigation bar you should be notify as to why you cannot change the url for this tab. Or, if you do edit the url in the navigation bar to change then the tab should not be pinned anymore.