Why did Mozilla Firefox decide to make the minimum window width to 450px?

I'm wondering why the browser Mozilla Firefox decided to set the minimum sizable width of its window to 438px. Is there any particular reason for this? Because if I were to develop responsive website, Chromium browsers have minimum width of 500px.

How to replicate:

1. Open Firefox
2. Right click inside a website
3. Select "Inspect Element"
4. Click on the top right corner beside the phone icon to show more options
5. Select "Separate Window"
6. Resize the browser
7. Note: Remove the scrollbar

This thread should explain this value: https://bugzilla.mozilla.org/show_bug.cgi?id=897160

TLDR; Shrinking the window "too much" would cause UI bugs, so it was decided to set a fixed minimum browser width