This is exactly what the extension "Add to search bar" is for.
this is the developers homepage with a video how to use it: https://firefox.maltekraus.de/extensions/add-to-search-bar
to install it you need "Palemoon's Classic AddOn Archive"
read here https://forum.palemoon.org/viewtopic.php?f=46&t=17294
newest version here:
The AddOn Archive allows you to install all the XUL Addons that Mozilla deleted from their homepage
> I don't know if it's relevant but I use "instantfox" as well.
Yes, it appears Instantfox is broken. You could have easily tested this by disabling it on your addon menu (about:addons or Ctrl+Shift+A). I recommend you disable it.
To maintain most of that addons functionality, you can use Add to Search Bar to add custom searches, and add keywords. This will allow you to enter things such as 'g whatever' to Google "whatever", but it won't be instant.