Mouse back and forward buttons doesn't work in Firefox

[samhed]

Describe the bug
With #1919 support was added for back & forward mouse buttons in noVNC. These do not work in Firefox, the browser consumes the events, causing you to go backwards or forwards inside Firefox instead. The idea is that these events should be sent to the server, letting the remote application decide what should happen.

The issue was reported to Firefox here:

https://bugzilla.mozilla.org/show_bug.cgi?id=1933746

This is a tracker bug on our end, there is not anything we can do until Firefox fixes this. Our code should be prepared for the day that happens.

To reproduce
Steps to reproduce the behavior:

1. Connect to a TigerVNC server using Firefox
2. Click the back-button on your mouse
3. Observe that your browser will navigate backwards, instead of letting the mouse event be handled on the server.

Expected behavior
These mouse clicks should go through to the server. Most applications handle them as back and forward actions.

Client (please complete the following information):

• OS: any
• Browser: Firefox
• Browser version: 133

Server (please complete the following information):

• noVNC version: master
• VNC server: TigerVNC
• WebSocket proxy: websockify

Additional context
Server-side support for this feature is available in TigerVNC.