Maybe with this Add-on that adds a "Open link in current tab" to the right-click context menu - https://addons.mozilla.org/en-US/firefox/addon/ope
If you are referring to using context (right-click) search with your default search bar engine after highlighting a word or phrase and having the results overwrite the current tab from which you are searching, try this extension (the instructive material on the add-on page is a bit "foggy" on whether it will open the results in the same tab and overwrite the page from which you are searching) - https://addons.mozilla.org/en-us/firefox/addon/con
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