Thanks @Shivam, your solution fixed the issue of this add-ons in MacOS :)
~~~ Terminal
> echo '{"browser_binary": "/Applications/Firefox.app/Contents/MacOS/firefox"}' >~/Library/Preferences/ax.nd.nulldev.FirefoxProfileSwitcher/config.json
~~~

Fix in MacOS: "echo '{"browser_binary": "/Applications/Firefox.app/Contents/MacOS/firefox"}' >~/Library/Preferences/ax.nd.nulldev.FirefoxProfileSwitcher/config.json"

Worked great, broke, but then followed Zein Adi/olivrrstray's instructions about creating a config.json and now working again. Thanks!

usually is very helpful, but it just randomly stopped working for no reason???? please explain.

Is your extension broken? Go to this review: https://addons.mozilla.org/en-US/firefox/addon/profile-switcher/reviews/2057565/

Stopped working but followed @olivrrstray instructions below and it is working again. Thank you.

Does not work. You can click profiles but it won't switch to them or load them. Nothing happens.

It was great while it worked. Now it no longer switches profiles. I will uninstall. The workaround I use now is to add the -p option to firefox.exe in the shortcut. That way every time I wish to switch profiles, I just open the Firefox icon on my desktop and it will ask which profile I want to use.

Currently down, but olivrrstray in the reviews has a fix for windows that was easy to follow, and when working properly it's fantastic, very easy to use and i love having multiple profiles available

This is a fantastic add-on that really helps me use Firefox in the multiple ways I need to without having to log in and sync multiple Mozilla profiles, however, it's currently malfunctioning (23-Mar-2024). Thanks to olivrrstray's review I was able to get back in and I concur with JavitoPlay, the dev is most likely working to update it.

worked well before but is broken now

Now its broken the "click" on the profiles, but you can go to edit profiles, and "set as default" to open the profiles you had keep.

I suppose the developer is working to fix it

In general, this extension its great.

Broken

Broken with the March 19th update, 124.0.

Edit:
For the more tech savvy, there is a workaround on Issues tab on the Github page.

For the less tech savvy who have Windows:
1. Hit Windows + R.

2. Put "%APPDATA%\nulldev\FirefoxProfileSwitcher\config\" in the prompt, and hit "ok." This will open the folder with configuration files.

3. Copy the path; this is the address in the top bar. It should look similar to this:
C:\Users\[yourusername]\AppData\Roaming\nulldev\FirefoxProfileSwitcher\config

4. Open the notepad app, create a new file. Paste the following text in the notepad:
{"browser_binary": "C:\\Program Files\\Mozilla Firefox\\firefox.exe"}

5. Open the File menu, click Save As.

6. In the address bar, paste the address you saw before, and change "Save as type" from Text documents to all files. You should see files like "avatar.json"

7. Save the file as: config.json

8. Reopen Firefox

Was working well but now unresponsive, cannot access my other profile.

Update - fixed it thanks to olivrrstray's short guide.

It was working fine but recently my clicks have been non responsive. I'm locked out of my other profile and am unable to access any of my bookmarks

It worked fine until Firefox version 124. Now it doesn't react on when I want to switch profile.

GOOD

Took a minute to setup, but once done its an awesome experience! btw im on a Macbook Pro 2019 Intel I7

Works flawlessly on m2 pro macbook running Sonoma, honestly insane that this hasn't been implemented natively yet

Unclear and confusing in Fedora