I kept checking back to the reviews in hopes of more info on the installation of "open with", and there it was. I put open_with_windows.py in my Windows\system32......opened a command prompt, which opened in the Windows\system32 folder, and ran the open_with_windows.py install, and that was it for me. I ran test installation in my Browser and Thunderbird, and it showed, "Found version 7.2.3 at C:\​Windows\​System32\​open_with_windows.py." I then edited the order of my four Browsers, with Firefox on top.
Thanks to all the reviews with the help, and thanks to the the Author of this addon!!

Amazing addon! I'm only missing the ability to select multiple tabs and open them on the chosen browser, instead of doing it manually for each tab! There's an open issue on the github repo asking for this already, just in case other users would like to show their interest on this feature.

Misleading description, This extension can also launch different application and media player . Superb

it works perfectly with firefox and chrome. But in Firefox Developer Edition , it can't detect browsers.

Useful extension but unfortunately it requires few extra steps maybe not easy to understand by basic users.

Donation made. Thank you 👍

If you are on macOS Catalina, install this extension and follow the instructions, but put the file "open_with_mac.py" in your "Public" folder. Only then it will work.

For people who cannot use it on macOS Catalina. I found a solution here, https://stackoverflow.com/questions/59543692/failed-to-connect-native-host-has-exited . In other words, download the helper program open_with_mac.py to $HOME/Public .
For debugging, I find it easier to debug with Chrome browser. There is a Chrome version of this extension https://darktrojan.github.io/openwith/

I was going to give 1 star. Till I realized I was in older python folder on main C Directory was wrong, I removed older versions and Launcher of Python from control panel (Programs and Features). and reloaded Python version 3.8.1 and noticed where it was loading it (Windows) >
C:\Users\xxx\AppData\Local\Programs\Python
I dumped the "open_with_windows.py" File downloaded from (On the Option Page when you click the toolbar button) I placed that file after downloading it in the Python folder and then Ran the "open_with_windows.py install" in Command Prompt Admin THEN Tested it via > Options on the Open With toolbar button got a green check and then it worked when I tried it on a web site page.

Excellent.

Installed the add-on & helper on my Linux system, and found it very easy to manually add a lightweight browser (Qupzilla) to the list.

Open With saves me a whole lot of hassle. Some sites don't work properly in FF, so, using Open With I can quickly load the current page in Chrome or (perish the thought!) Edge. But still more useful and indeed important for me is that I can quickly load the current page in BlueGriffon, my web page editor, or in EditPad Pro. I use both these latter for working on my local version of my extensive five websites. Saves a huge amount of hassle!

One odd thing. Just lately, the toolbar icon for Open With has taken to repeatedly giving a warning message, that Open With couldn't make contact with the outside world (presumably meaning it couldn't make an Internet connection). But its permissions don't include being able to do so, so something is clearly screwy about this.

As another reviewer has remarked, it would be greatly helpful to have a facility for exporting / importing one's settings.

Update:
I found that this extension had ceased working, and presumably this was associated with the error message noted above. I followed a reasonable hunch and re-ran "[path]open_with_windows.py install", and that fixed it.