https://webprog3.com,sametab
Using webprog3 to create static web pages should present no issues. This entire website was produced using webProg3 with no significant trouble. However, once you begin including Javascript/JQuery to create dynamic pages, you might, for example, make mistakes in the coding such as typing errors, wrong syntax, and so on, and as a result your page will not respond as you expected. If you cannot immediately identify the reason, then Firefox can, perhaps, give you some help in making the diagnosis.
For a simple demonstration, as I am preparing this very webpage, I am going to open up the Code Editor and type in the word "gobbledygook", and then do a Preview of the page.
Then let's see what Firefox has to say about it. Here we go ........
If you look below, you will see that the page displays OK, in spite of the rubbish in the Javascript: there is nothing wrong with the HTML.
So let's look in the menu. Click on the "triple-bar" icon in the top right-hand corner.
In the menu, you will find "Web Developer".
That's you, so click on it.
And there it is. Firefox doesn't like your gobbledygook!
Of course, not all diagnoses are as clear as this one. You can type a comma instead of a full stop for example, and Firefox might accuse you of all kinds of crimes!
Relying on Firefox's diagnoses alone is often not sufficient for effective debugging. Personally, my scripts, while under development, are absolutely peppered with Javascript alerts so that I can report the values of variables and check that they are what they should be. Or if alerts fail to appear, I can deduce at what point the script has failed.