The addon is now archived (no more updates).
If it breaks in the future try the following alternative addon:
External Application Button
(by Andy Portmen)

I heard that the "Open With" developer no longer maintains the extension (I hope that some developer continues to update and maintain the extension, as it is very useful to open YT-DLP and video players such as MPV, PotPlayer, VLC and MPC- HC)

A great app! Has somebody found a way to use it with Snap Firefox?

Only for Unix geeks. I'm no newbie, but have tried absolutely everything on here and it simply will not install. Latest version of Python? Check. ---Terminal commands: can only get it ti recognise if I drag the .py file onto terminal. Then, install sill not run no matter the combo. 'Command not found'. This should be part of T-Bird's BASIC setups.

Open With is one of the few remaining Firefox extensions that are both extremely useful and actually working as designed. After downloading, follow the steps in the Open With Options page to install it.

First, use the "Fetch it from here" link to download the latest version of the Python software. Click on the downloaded installer to install Python. By default, it will install to C:\Users\[username]\AppData\Local\Programs.

Second, use the "Click here to download" link to download the Python file opem_with_windows.py. Move the file from the Downloads folder to the folder where you want it to be installed.

Third, use Command Prompt to install the .py file. It's a 2-step process. First, you have to tell Command Prompt where you put the file. Then you have to tell Command Prompt to install it.

For example, if you moved the .py file to C:\Users\[username]\Firefox, then the first command you would type in Command Prompt would be

cd "C:\Users\[username]\Firefox"

Press Enter. Then type

open_with_windows.py install

Press enter. Close Command Prompt.

Now, in the Open With Options page, click the "Test Installation" button. If a green check mark appears, you're done. If a red "X" appears, repeat the steps above until you get it right.

IMPORTANT NOTE: Understand that the Python software you downloaded enables the Python .py file to function, and the .py file enables the Open With extension to work. If you rename or move the folder in which you installed the .py file, the extension will stop working. In that case, use Command Prompt as above to reinstall the .py file in the new or renamed folder location.

V 7.1 (Python 2.7) stopped working after upgrade to Firefox 100.0.2 from 99.0.b8.

"Something went wrong. There might be more information in the Browser Console.."

Any ideas?

I've looked thru the reviews for this extension to figure out how to install it and none of the suggestions have helps. When I run the cmd line I get the same result every time:
C:\Users\metal321\OpenWithScript>open_with_windows.py install
This is the Open With native helper, version 7.2.6.
Run this script again with the word "install" after the file name to install.

I found this extension: Open in Google Chrome Browser
https://addons.mozilla.org/en-US/firefox/addon/open-in-chrome-browser/#:~:text=%22Open%20in%20Google%20Chrome%22%20extension,manually%20copy%20and%20paste%20links.

It works by simply installing it and there are a number of other
extensions for other browsers and programs listed here:
https://add0n.com/open-in.html?from=chrome&version=0.2.8&type=install

I found the extension great, but I can't install the script.
I type the path in cmd with the script name and then install, but cmd keeps telling me to type install.

C:\Users\User\AppData\Local\Programs\Python\Python310>open_with_windows.py install
This is the Open With native helper, version 7.2.6.
Run this script again with the word "install" after the file name to install.

I have no idea where the error is.

This extension is absolutely amazing. I consider it one of the very select "Absolutely Mandatory and Indispensable" extensions I always use. I truly do not know how I lived without it, before first finding it. I don't know why Mozilla isn't pushing this as one of the *Recommended* extensions, because it should. Maybe I'm overestimating the number of people who would be interested in launching a webpage from one browser directly into a specific other browser, but for those that ever do this more than occasionally, this thing is terrific. I love Open With so much.

A little messy to install, but very useful when you get it going.

Works perfectly on Manjaro Linux! The AMO page should mention that the extension requires additional setup after installation. It took me a while to figure out that I need to go to the Options page after installing the extension.

AWESOME! I wanted such an extension for a long time! The best part? It supports Linux, unlike some other similar extensions! :D
This extension has a HUGE potential! I just added SMPlayer to watch YT vids in it and damn - it works! :D :D

Useful to run commands like mpv or yt-dlp. I've quickly reviewed parts of the source code, looks fine.

For the above commands, best practice is to add "--" as the last argument. (Maybe not a real concern, because the URL is passed as a single argument and probably cannot start with "-", but just in case.)