I used your custom textarea control code and applied it a field in a sample locally.
I see the label as well as the textarea field on load of the harness in Firefox. I'm using 45.0.1
It is consistent with IE as well...
OnBlur/Post value also works in both browsers.
To continue helping you on this, can you please do the following -
1. Do the above test with your custom code on PegaSample application rulesets. Create a temporary access group(that uses OOTB User portal) with a test ruleset, let your operator point to it, override the OOTB TextArea control code with your code, apply it to a field and in an OOTB flow like PegaSample-Task and run it in FF and IE 11. Confirm that you still see the issue.
2. Collect Fiddler data when you load your harness in IE and FF and attach that evidence here.
3. Open the pega_ui_textarea.js rule, go the history tab - tell me the ruleset name/version# and update timestamp details of this rule.