Per the screenshot here - https://addons.mozilla.org/en-US/firefox/addon/reverse-image-search-dmtr-org/ - it appears that there is a contextual menu item to do the search at Dmtr.org .

Also says Right click any image to perform search using new tabs.

Hi   !
That must be such a disappointment   ! This one has very good reviews;   hopefully it will work for you as well:
https://addons.mozilla.org/en-US/firefox/addon/google-reverse-image-search/
Keeping my fingers crossed   .........

@the-edmeister No, it turns out it's just really fussy where you click a pic to get the option to appear in the context menu. I guess it's a work-in-progress. Thankyou for trying to help

@Happy112 Thanks, but unfortunately the reason I chose this one is it uses multiple search engines. Google censors its results, to do with copyright law I believe, so on Chrome I use one called RevEye which searches on baidu, yandex etc as well as google. This one is the only equivalent I could find on Firefox. It does work, just needs a lot of trial and error right-clicking the different parts of a pic until the option appears in the context menu. Hopefully the person that made it will fix this at some point