Yes, most likely, it's a problem with form control styling in multiprocess Firefox.

Are you using e10s?

I'm pretty sure you are based on your symptoms, but you can confirm you have this feature turned on as follows. Either:

• "3-bar" menu button > "?" button > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter

In the first table on the page, check the row for "Multiprocess Windows" and see whether the number on the left side of the fraction is greater than zero. If so, you are using e10s.

With e10s active, the "options" that display in the drop-down below a select control may be rendered using a default font instead of the one assigned by the page. However, I haven't seen enough examples to know the pattern of what font is used.

If you are using e10s:

To work around this issue, you could turn it off as follows:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful.

(2) In the search box above the list, type or paste autos and pause while the list is filtered

(3) Double-click the browser.tabs.remote.autostart.2 preference to switch the value from true to false

Note: the exact name of the preference may vary, but it will start with browser.tabs.remote.autostart

At your next Firefox startup, it should run in the traditional way. Any difference?

Why is it okay after a Refresh, but only until the second session?

When you first install or after you refresh Firefox, it starts up traditionally and then performs a check to see whether it can use the new e10s feature starting with the next startup. That's why you see the change when you see it.

By the way, the developers are mulling a solution to this. It might take a couple versions before it is finally released.