Usually these lists are 10 best, but really, for Rails developers, there are only two plugins you really need.
DevBoi plugin, with Rails addon
The Rails addon pack gives you Rails and Ruby documentation too. Ctrl-F9 (on Windows) brings up the sidebar.
I can’t work without this plugin.
Install it from here: http://www.martincohen.info/products/devboi/
This plugin allows you to view all sorts of useful information about the current page. When you’re building pages, and nothing seems to be working, the information this plugin provides is invaluable.
Get the Web Developer plugin here: https://addons.mozilla.org/firefox/60/