Controlling the browser from command line

Question

I recently switched most of my work from Mac (back) to Linux - with no regrets.

The one thing I still need to figure out is how to control a browser via scripts from the command line.

Primary purpose is to control the currently running browser to fill in forms that I use repeatedly (i.e.: bank payments).

This is something that I used to do via AppleScript+JavaScript on Chrome and before that via perl+MozRepl on Firefox.

Key requirement

• the ability to read stdin - directly or not (via bash/perl scripting is ok)
• the ability to fill in forms (NOT via URL)
• NOT headless (this is for bank payments)

AppleScript is not available on Linux (duh!) and MozRepl has been deprecated a couple years ago.

Any (lightweight) solutions?