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