Chrome Version       : 64.0.3282.167
URLs (if applicable) :
Other browsers tested:
       Safari: not tested
    Firefox: FAIL 59.0.2 (windows 7, windows 10, ubuntu 16)
       Edge: PASS 40.15063.674.0

What steps will reproduce the problem?
I don't have simple steps. Basically use hterm in a web page displayed in firefox. Have some text in the <div> given to TerminalEmulator.decorate(). Have ctrl-c-copy and ctrl-v-paste set to true in PreferenceManager. Select some text using the mouse and observe nothing lands in the clipboard (try pasting into notepad or wordpad), and NS_ERROR_FAILURE appears in firefox javascript console.

What is the expected result?
When I use ctrl+c to copy text highlighted inside the hterm pane, the selected text should be copied into the clipboard and thus available when I use ctrl+v to paste (either into the hterm pane or into other applications).

What happens instead?
Firefox does not copy anything into the clipboard. Javascript console shows NS_ERROR_FAILURE (no other text appears, just NS_ERROR_FAILURE) with a line number that points to the function hterm.Terminal.prototype.copyStringToClipboard() at the line:
selection.selectAllChildren(copySource);

Please provide any additional information below. Attach a screenshot if
possible.
I searched around and found known issue in mozilla bug database that seems to match this problem. It's 3 years old, so it's unlikely to get fixed:
https://bugzilla.mozilla.org/show_bug.cgi?id=1178676

I searched around for other ways I could select text (which is what the selectAllChildren() is doing) and found that I could replace that one line of code with this code block:

    var range = this.document_.createRange();
    range.selectNodeContents(copySource);
    selection.removeAllRanges();
    selection.addRange(range);

This eliminated the firefox exception and successfully copies to clipboard in firefox (tested on windows 7, windows 10, ubuntu 16), chrome (tested on windows 7, windows 10), and edge. I am not super expert, so maybe this is or is not the right solution. Just wanted to provide something as a starting point.