Any external link can trigger such things, so even images and such in sigs can cause problems. Sometimes I have had images(on official forums at least) force a pop up window that asked for login/password information.

edit: Did a search for said url in signatures, came up with nothing *shrug*

Reports like this can be contested with Google if you find they are reporting false positives.

On the other hand, typically all current browsers draw malware information from a single source(Google) so if you're getting the report then everyone else, Chrome and Firefox users alike, would be getting the same warning. Does this pop up only on specific threads? Is there any way you can provide more details(usually in the form of "why are you showing me this?")?