How to trace what is slow-down site loading?

Problem summary:

  1. I’m entering a address page website URL and pressing enter
  2. Waiting for around few seconds or some amount of time (not natural) before the page will start to load
  3. I don’t know what is affect the issue and how to debug and trace it to understand what is wrong and how to fix it.
  4. I need from the community a mini-howto for fixing the issue.

Long story
I love firefox, for font rendering and other features. But after some time at latest actual firefox version (and not only), I frequently notice the issue which affects site loading. Looks like somewhere at a point of resolving DNS name or somewhere near that happened some strange issue which slowdown whole browser performance, and the worst thing, that I don’t know how to trace the issue and find out what is a reason of the slow site loading. Because my computer resources not at 100% usage, I have very good latency over the world, and stable internet connection, I have powerful pc, and latest actual up to date browser, operation system, drivers, and so on.

At the same time, I do not have any issues with any other browser, like chrome, opera, etc.
The issue looks like you waiting for something before the rendering process begins. And only when started website rendering the whole site loading pretty fast.

Please do not suggest me next:

  • antivirus related suggestions (I’m very old PC user with big experience)
  • enabling/disabling addons and so on
  • resetting profiles
  • debugging my internet connection
  • checking to resource monitoring
  • reinstalling Firefox
  • updating drivers
  • scanning for viruses
  • updating operation system
  • and any other newbie things which are NOT related to the problem.
  • process contents/hardware acceleration / different options at GUI with firefox

I’m looking for an instrument and mini howto for tracing and debugging such issues. .What does means “debugging”, and how am I imagine this? Easy! I see that some site is slow in loading, I’m pressing some and site loading profiling record begun, then I’m repeating the same thing on different other sites, and can view what is going on.

Then, when the data will be collected, I must to know, where do I need to send this collected data for further updates and bugfixes for the Firefox (to make it instantly fast)