The intended use case is when you suddenly realize you have more tabs opened in Firefox than the browser can handle without slowing to a crawl. Chrome has no problem handling a lot of tabs and as such is better suited for managing a lot of them. This plugin provides an easy way to migrate your opened tabs off Firefox onto Google Chrome, so you can close down the tabs in Firefox and make the browser usable again.
How to get the Clickable icon seen in the screenshots
In Firefox, simply right-click somewhere outside the web page area and select the Customize menu option. This will bring up a lot of icons, so scroll through that list until you see the addon's icon. Then drag-and-drop the icon where you want it on the Firefox screen.
By default left clicking the icon opens the current tab in Chrome, and CTRL+left click opens all your Firefox tabs. The key and button combinations are customizable in the options dialog.
This addon extension is living on borrowed time. When Firefox 57 is released, the addon will cease to work, likely along with most of your other treasured addons. The reason is that Mozilla is removing the ability for addon developers to innovate by interacting with the operating system. When Firefox 57 drops, Mozilla yanks the rug from under this addon and all other addons I currently depend on for my personal use, which means I plan on moving to another browser at that time and consequently won't be making any further updates to this extension.
I'm sad to let this go, and create a gap in other people's browsing workflow, but with the current Mozilla plans, there really is nothing I can do. Mozilla has made it clear they don't want to offer a platform which allows for innovation, but prefers to converge towards the limitations of Google Chrome.
There's a reason there's been no "Open in Firefox" addon for Chrome, it's technically not possible with WebExtensions without either requiring the user to install additional programs on their machines, or send your private data to external servers. Mozilla is aiming to produce a 100% compatible clone of Chrome, warts and all.
It makes no sense for me to continue supporting Firefox when the sole purpose of this browser's existence is that it presently allows developers to create addons which do things impossible in Chrome, and when that reason goes away, so does the justification of Firefox. That's the underlying reason for this announcement.