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Windows Vista is no longer a concern, so find_windows_browsers() doesn't have to worry about the KEY_WOW64_* flags. IMO, it should get the browser's real name (the default value of the key) and the fully-qualified path of the executable, instead of depending solely on an "App Paths" entry being configured for the base executable name. For example:
 def find_windows_browsers():
 """ Read the installed browsers from the Windows registry."""
 import winreg
 browsers = []
 with winreg.OpenKey(winreg.HKEY_LOCAL_MACHINE,
 r"Software\Clients\StartMenuInternet") as hkey:
 i = 0
 while True:
 try:
 subkey = winreg.EnumKey(hkey, i)
 i += 1
 except OSError as e:
 if e.winerror != 259: # ERROR_NO_MORE_ITEMS
 raise
 break
 try:
 name = winreg.QueryValue(hkey, subkey)
 if not name or not isinstance(name, str):
 name = subkey
 except OSError:
 name = subkey
 try:
 cmd = winreg.QueryValue(hkey, rf"{subkey}\shell\open\command")
 cmd = cmd.strip('"')
 os.stat(cmd)
 except (OSError, AttributeError, TypeError, ValueError):
 cmd = ""
 browsers.append((name, cmd))
 return browsers
The loop over the result would change to `for browser, cmd in find_windows_browsers()`. The string to match for Internet Explorer, using the real name instead of the registry key name, would be "internet explorer". A class for Microsoft Edge ("msedge") should be added.
The browser would get instantiated with the cmd value, which ideally is the fully-qualified path of the executable. The fallback behavior wouldn't change for the case where self.cmd is an empty string. For example:
 class WindowsDefault(BaseBrowser):
 cmd = newwindow = newtab = ""
 def __init__(self, name="", cmd=""):
 super().__init__(name)
 if cmd:
 self.cmd = cmd
 ...