How can I troubleshoot slow Firefox page loading on some websites?

I sometimes notice that pages take longer to load in Firefox, even when my internet connection seems to be working normally. The issue does not happen on every website, so I’m trying to understand how to identify whether the cause is Firefox itself, an extension, cached data, DNS, or the website/server. What Firefox settings or [http://example.com troubleshooting steps] should I check first to determine what is causing the slow loading? I have already tried refreshing the pages and checking the connection, but I would like to follow a more systematic troubleshooting process.