If you used the Developer Tools built into those two web-browsers to Inspect the relevant section of the HTML generated for your example you will see that it looks something like the following.
<tw-passage tags="">
<tw-sidebar>...</tw-sidebar>
Here is some poetry:
<br>
<br>
stanza one line one
<br>
stanza one line two
<br>
stanza one line three
<br>
stanza one line four
<br>
<br>
stanza two line one
<br>
...
</tw-passage>
Harlowe includes the following default CSS rule...
br + br {
display: block;
height: 0;
margin: 0.8ex 0;
}
... which is meant to only effect the situation where you have two consequential line-breaks (BR elements) and there is nothing between those two line-breaks. This is known as an Adjacent sibling combinator selector.
Ideally this rule should only affect the double <br> elements between "Here is some poetry:" and "stanza one line one" and the ones between "stanza one line four" and "stanza two line one", and in the case of Chrome this is true because all the other <br> elements are separated by a line of text.
However Firefox also treats the <br> elements are separated by a line of text as if they are the same as the double <br> elements. This results in the 0.8ex top & bottom margin being applied to all the <br> elements, which in turns results in extra space between each line of a stanza.
note: You can test this yourself by Inspecting the HTML of the current page in Firefox and temporarily disable that margin in that CSS rule, you will then see that the extra spacing disappears.
I would guess that the reason this is happening is because those lines of text are what's known as Text Nodes, which are slightly different to a HTML element, and for some reason known to only the developers of Firefox they are not considered when determine if a CSS Adjacent sibling combinator selector rule should be applied or not.
There are a number of ways to fix this issue, one of the simpler ones being to wrap each of those lines of text within a HTML element like so. (you don't have to use a <span> element, you can choose one of the others.)
<span>Here is some poetry:</span>
<span>stanza one line one</span>
<span>stanza one line two</span>
<span>stanza one line three</span>
<span>stanza one line four</span>
<span>stanza two line one</span>
<span>stanza two line two</span>
<span>stanza two line three</span>
<span>stanza two line four</span>
<span>stanza three line one</span>
<span>stanza three line two</span>
<span>stanza three line three</span>
<span>stanza three line four</span>
... now there is a HTML element between each of the single <br> elements. so those single <br> elements will no longer be affected by the double <br> element CSS rule.