Send iMessage from Firefox Ubuntu + Firefox has become my daily driver. I would like to be able to send iMessage from Firefox. I am able to login using my Apple ID in Firefox. Is there a URL that can enable me to send iMessage from Firefox?
No. iMessage (rebranded as Messages) service is only implemented on Apple hardware and it is not available for other desktop/mobile/web platforms. Apple does offer web apps for other parts of iCloud but not iMessage or FaceTime.

You need an Apple device, specifically running either iOS (iPhone, iPad, iPod touch), macOS (Mac desktops and MacBook portables) or watchOS (Apple Watch) to access Messages via native app available for each platform.

Messages is not accessible using other Web browsers such as Firefox, and not available even using Apple's own Web browser Safari.