firstLineIndent can’t be set to zero
Create a text object, type some text, keep it selected and run the script:
activeDocument.textFrames[0].paragraphs[0].paragraphAttributes.firstLineIndent = 20;
alert(activeDocument.textFrames[0].paragraphs[0].paragraphAttributes.firstLineIndent);
activeDocument.textFrames[0].paragraphs[0].paragraphAttributes.firstLineIndent = 0;
alert(activeDocument.textFrames[0].paragraphs[0].paragraphAttributes.firstLineIndent);
You have two alerts saying 20, while the second one should show and actually set the first line indent value to 0 — and it does not.

-
[Deleted User] commented
Transforming the text frame before changing the indent property helps to fix the bug. But tricks don't make writing code any easier. The same trick helps to fix ".textRange.justification = Justification.LEFT;" bug.
-
[Deleted User] commented
Example