The ECMA specification (ECMA-262) dictates acceptable regular expression grammar and multiline support is explicitly mentioned. Section 15.10.2.6 in particular demonstrates the effect of multiline support on Assertions.
So, if the browser supports the ECMA-262 specification, then yes, it supports multiline.
Of course, you should endeavor to test your code before you can be confident it works. It wouldn't be the first time a browser has failed to implement a specification correctly.
EDIT:
To clarify, JavaScript 1.5 is fully compatible with ECMA-262, Edition 3. I understand the following browsers support JavaScript version 1.5 or higher:
- Safari 2.0 or newer (Mac)
- Camino 0.8 or newer (Mac)
- Firefox 0.9 or newer (Windows, Mac, Linux)
- Internet Explorer 6.0 or newer (Windows)
- Mozilla 1.2 or newer (Linux)
- Netscape 7.1 or newer (Windows, Mac)
- Opera 8.0 or newer (Windows, Mac, Linux)
Can't actually find a decent source for Chrome although I'd be shocked if it didn't support JavaScript >= 1.5.