> For me it's a simple feature in Chrome "tab to search" which frustratingly has never been ported into Firefox

Firefox has a similar feature called 'Quick Searches' (look in Firefox's Bookmarks menu). For example, I have one for Flickr - keyword 'fl', url 'https://www.flickr.com/search/?q=%s&w=all'.

So if I type 'fl bridges' in the address bar I go to https://www.flickr.com/search/?q=bridges&w=all