It's site-dependend and system-dependend. For wikipedia I've ctrl+shift+f, but it can be different for Mac ;).

ctrl+shift+f doesn't work on Windows 8 - dark ages work laptop they gave me. Anyone know the answer for Windows and Linux/Ubuntu?

https://addons.mozilla.org/en-US/firefox/addon/add-to-search-bar/ Add to Search Bar Make any pages' search functionality available in the Search Bar

https://addons.mozilla.org/en-US/firefox/addon/context-search/ Context Search Expands the context menu's 'Search for' item into a list of installed search engines, allowing you to choose the engine you want to use for each search.

Sorry, neither of those plugins do what I was looking for.

It might require a standard, formal or informal, where something is included in the html to mark where the search box is on the page. Then a shortcut in a plugin could take you straight to the search box on the page.

One adds that sites search to the Firefox search box. You don't need to be on the site to use it.

The other, when on a website, will use that site's search box without you having to look for it.

You could create a bookmarklet to set focus to a specific field and use an extension to add a keyboard shortcut. I have such a bookmarklet to set focus to the textarea on this forum and some others.

It is basically a list of queries and setting the focus to the element when found.

For the search bar on this page:

javascript:(function(){var e=document.querySelectorAll('input[id="search-q"]');if(e.length){/*console.log(e[0].nodeName);*/e[0].focus();e[0].scrollIntoView(true);}})()