Unable to change Username Using Firefox

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I would like to change my username/URL slug on Github. However, when I followed the instructions to do so on my profile (by clicking the Change Username button on the appropriate page), I was not shown the warning dialog/modal. However, the following appeared in the console/debug window when using "Inspect Element" from the context menu:

Uncaught ReferenceError: HTMLDialogElement is not defined
    tM dialog_helper.js:23
    connectedCallback dialog_helper.js:84
    38168 dialog_helper.js:118

For reference, I'm using Mozilla Firefox 88.0 for Ubuntu on Ubuntu 16. (Yeah, I know, it's outdated, but so is my laptop.)