OMFG it was a permission for the Firefox app itself, in android: https://support.mozilla.org/en-US/questions/1316457
Not sure how/why I turned that off (I'm just going to assume it was me at this point), or why the file upload button does nothing, and gives no feedback, if you don't have on the 'camera' permission, but glad another months long problem for me on ff is finally resolved.
OK, but here's a kicker: even if you've denied access to Firefox for the camera permission, it's possible to bypass that, and still open the file dialog, without actually getting camera permission. https://support.mozilla.org/en-US/questions/927165
Which was an embarrassing exchange where two people completely talked past each other. But there's a jsfiddle where I get and close the alert, click the browse button, and the file dialog comes up even tho I have Firefox set to deny camera permission and not to ask me anymore.
Basic html input element: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input/file
Jsfiddle (I think*) using js click for file upload, but doesn't bypass permissions: http://jsfiddle.net/NkycS/2/
Jsfiddle for bypassing camera perms after they've been denied: http://jsfiddle.net/3wJSY/
* I say "I think" because neither MDN's page nor jsfiddle are very mobile display friendly, and another mobile problem I'm having**.
I may create simple reproduction pages on my site with steps and pass this along to Mozilla, if they care. ... ***
** My other mobile Firefox problem is that I can't seem to usefully type in any sore of more than plain text box. What I mean is, any text area on mobile firefox where one would edit code, completely freaks out for me. Sometimes deleting doesn't work, or my cursor jumps to different points in the text as I'm deleting or typing. Basically completely unusable.
*** If I'm not too tired and cranky from staying up too late investigating this on my phone, I'll try and reproduce and pass along to Mozilla/Firefox team via support? Reddit? Idk.
But if I make those simple reproduction pages, I'll have to paste the escaped source code in my post, or else the dev team won't be able to read it on mobile. Because FIREFOX MOBILE DOESN'T SUPPORT VIEW SOURCE.
I even tried a JS bookmark hack. But you can't add custom bookmarks on mobile either!
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