Which browser and what OS? They put things in different menus depending on those two details(instead of making them consistent everywhere).

Go find your settings panel.
Set your "home page url" to about:blank (or whatever site you want)
Some browsers like Firefox let you set your new tab url separately.
You might also want to scroll down to "url suggestions and uncheck a few things" so that as you type its not literally everything you've ever browsed as the autocomplete suggestions.

I think the OP is referring to Steam -> Settings -> "In Game" -> "Web browser home page;" the setting is indeed unchangeable.

Practically speaking, since Steam's browser lacks a "home" button or a means of starting it with an unspecified URL--except when adding a tab--it makes little difference. It lacks too many features to be used in place of any fully functional browser.

I have mine set to about:blank and it works exactly like I wanted it to.
Its changeable and I've always been able to change it.
Does the vertical bar indicator(meaning insert mode) not show up? You can also highlight consecutive words and start typing to replace those.
In what way is it not changeable for you?
Anyway, I usually ctrl-click around so the first time the new browser window shows up its already going somewhere I wanted to go.
Oh, I see it now... it just gets set back to google.com as soon as I click another settings page and click back. Yeah, that seems like a bug.