All of a sudden, it seems to have corrected itself. I did nothing different. In an effort to solve the problem, I rebooted several times, but it didn't change the outcome. After posting this question, it worked. I guess it's like taking your car to a mechanic to fix a problem and there's nothing wrong when he looks at it.

Some thoughts in case it happens again:

Does the linked site require preexisting cookies in order to load the specific links in the workbook?

If Firefox is open and running in the background, with you already having visited that linked site, then any necessary session cookies are available when the new page opens. On the other hand, if Firefox was closed and it launches its first window when you use the link in Excel, there are no session cookies yet. This could be important if there are no persistent cookies, either, which could occur in any of these scenarios:

• Firefox is set to delete cookies when you close it, or
• Firefox is set to start automatically in a private window, or
• Third party software removes cookies when you close Firefox

Thanks for the response. I don't know if it requires pre-existing cookies or not - I assume it does. I haven't had this problem before (that I can recall), and I've been using this spreadsheet with dozens of hyperlinks for months. When I copied the hyperlink address and pasted it into the browser, I thought it would set a cookie. But when I clicked on the hyperlink immediately afterwards, it still didn't work. The problem seemed to resolve itself as soon as I posted my question. Next time, maybe I should try that first rather than wasting an hour or so chasing ghosts.