Using the Error Console is a very useful way to describe a problem to the Developer if a problem cannot be reproduced.

telega's written description is very useful.

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Please collect information about browser error messages:
* Enter "about:config" in the browser's address bar. A page with a "Filter" input field and a list of preferences appears. Enter "javascript.options.showInConsole" into the "Filter" field, single line will be displayed. If column "Value" contains "false" then right-click the line and choose "Toggle" from the context menu, so that value becomes "true".
* Open FF Tools > Error console menu. An error console window appears. Click "Clear" button there.
* Now do what you were doing to reproduce the error.
* Go back to Error console window and see if there are any new messages.

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Enter "about:config" in the browser's address bar.

Enter about:config in the address field and press enter.

This rather daunting page appears...But please continue

Click "i'll be careful I promise" :-)

A page with a "Filter" input field and a list of preferences appears.

These are the Firefox settings
It is good to be careful

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Enter "javascript.options.showInConsole" into the "Filter" field, single line will be displayed.

Typing in this line narrows down the search field

If column "Value" contains "false" then right-click the line and choose "Toggle" from the context menu, so that value becomes "true".

So right clicking on the line creates a small menu.
If you chose Toggle you will see that you can toogle the Value between False and True.

False becomes True

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Open FF Tools > Error console menu. An error console window appears. Click "Clear" button there.

This is very important as it clears the Error console of any previous errors, which could otherwise confuse things completely.

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Go back to Error console window and see if there are any new messages.

What it means is here, make the action where you are encountering problems and then immediately afterwards look at the Error Console.

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The following Extensions are helpful when using the Error Console

Console²

MR Tech Toolkit
(formerly Local Install)

Bottom Error Console
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One of the easiest ways of accessing the Error Console is to do it directly from Fast Dial, this can be done by entering:
chrome://global/content/console.xul
in the URL Field of a Cell

In Firefox 3.x in Fast Dial 2.x, I think you will find that it will also be necessary to add a custom image to the cell otherwise it refreshes endlessly.

Once set up this is probably the easiest way to access the Error Console.