While there is no native support for Firefox, it is easy to use PageSnip with FireFox with one of the I.E. Extension plug-in's for Firefox.
There are many such extensions and PageSnip does not endorse or provide support for any of them, but PageSnip has been tested with this one:
You can download IE Tab in FireFox at:
https://addons.mozilla.org/en-US/firefox/addon/1419
Once installed in FireFox you can activate IE Tab by right-clicking on your page.
Then you will see the IE Tab menu:
After the page loads in the IE Tab you can access all of the PageSnip features from the right-click menu:
Note: PageSnip automatically detects Firefox installs and makes that available as one of the launch options from the Snip Information button. This makes it easy to use Firefox to return to a URL where a snip was taken.