The Firefox developer tools have a page ruler feature that overlay horizontal and vertical rulers on a web page, and show the dimensions of the viewport (in pixels) in the top-right corner of the viewport.
The feature is helpful but has a major caveat in that the ruler and pixel dimension display is turned off with each refresh of the page, which leads to hundreds if not thousands of extra wasted clicks to turn the ruler back on after updating the project's code and refreshing.
Is there any way to keep the ruler featuer toggled on through refreshes?
As of Firefox 88 this is not possible, unfortunately. There is an enhancement request to make the ruler persistent, though.