Released versions can be downloaded from https://addons.mozilla.org/firefox/addon/open-with/
Beta versions are sometimes available from https://addons.mozilla.org/firefox/addon/open-with/versions/beta
Download from the Chrome store https://chrome.google.com/webstore/detail/open-with/cogjlncmljjnjpbgppagklanlcbchlno. This version also works in Opera.
To get a working version of this repo in your Firefox profile, clone it, then link it into your extensions directory as openwith@darktrojan.net and start Firefox.
git clone git://github.com/darktrojan/openwith.git
realpath openwith > [your profile dir]/extensions/openwith@darktrojan.net
To get a working version of this repo in Chrome/Chromium/Opera, clone it, then from the Extensions page, click "Load unpacked extension…". You'll need to update your host file and reinstall it: find and replace cogjlncmljjnjpbgppagklanlcbchlno with the ID shown on the Extensions page.
Good luck.
Please send a pull request. Here's some information that might be helpful.