Ubuntu – Is there an equivalent to FireShot for Firefox

Now, Pale Moon is available for Linux. More on this, here.

In addition to the previously mentioned Iceweasel, there's also IceCat, Swiftfox and Swiftweasel.

(If you're feeling adventurous, there's also Conkeror, which is basically Firefox for people who hate using a mouse.)

Flock was discontinued.

Finally, if you want, literally another Firefox installation, you could just download the binary package and install it to another location.

Edit for setting as the definitive answer:

As indicated, the intended browsers should satisfy two conditions: (1) support Firefox addons and plugins, and (2) run separately from Firefox, without interference.

Icecat fully satisfies these demands. More details in this complementary answer.

Swiftweasel also satisfies the demands, but in relation with an outdated version of Firefox.

Seamonkey has a limited support for themes but the most important extensions are supported (more here.)

For details and updates keep an eye on this complementary answer.

On recent versions of Firefox, there is no need for any plugins. This capability is built into the browser.

1. In Firefox, use Ctrl+Shift+F2 to open the Web Developer Console.
2. Type :screenshot --fullpage (this will autocomplete, so :s Tab --fullpage will work).

That will save the screenshot as a PNG file to your downloads folder. By default, the filename includes the date and time. You can use a different filename if you wish, simply by passing it in on the CLI: :screenshot --fullpage my_name.png.

This will also work on pop-up windows, but each window will need to be saved individually. I know of no way to combine them into a single screenshot. (You could, of course, do that in GIMP or another image-editing tool later.)