You can make it look like any feature in any UI is hidden by choosing the longest path to reach it, using many words to describe it despite the target audience already knowing this stuff, and making your windows as small as possible.
Moreover, that a developer tool is a bit hidden in submenus in a UI designed for nontechnical users is fair game.
Even considering this, right click > inspect or Ctrl+shift+k also gets you the web developer tools. Not that hidden.
And then usually the network tab is visible immediately, it is one of the first tabs unless you moved it towards the end (even then, usually all the tabs are visible; but it's nice you can order the tabs as you want, and that a scroll button exists for when your window is too small -- and if the web developer panel is too small because it's docked at the left you can resize it, dock it to bottom or undock it).
This stuff is pretty standard across browsers, it's not like Firefox's UI is specifically weird for this. I don't have ideas for improving this a lot, it looks quite well designed and optimized to me already.
And then no, ublock Origin and No Script can't help you optimize the size of the web page you are working on. You ought to unblock everything to do this. They are a solution for end users, who have very few reasons to use the throttle feature. And unfortunately for end users, blocking scripts actually breaks too much to be a good, general workaround against web pages being too heavy. I know, I browse the web like this.
Any web developer knows how to find the network tab of the web developer tools