Yes, focus ring behavior is inconsistent in the latest version. It's probably a bug¹. As a workaround, you can perform the following steps:

1. Type about:config in the address bar and press Enter.
2. Press the big button to bypass the warning.
3. In the search box, paste browser.display.focus_ring_on_anything
4. In the search results, double-click browser.display.focus_ring_on_anything to set its value to true.

¹ Bug 785683 - Inconsistency behavior of focus ring after switching Fullscreen with F11 key.

Now it shows the box, but on darker background is really hard to see the box (dots). How to make the color brighter when there is dark background.

I don't see any preference for changing the focus ring color.

• browser.display.use_focus_colors affects whether the following custom focus colors are used.
• browser.display.focus_ring_style affects whether the focus ring is dotted (1 - default) or solid (0).
• browser.display.focus_ring_width affects the thickness of the focus ring.
• browser.display.focus_background_color affects the background color.
• browser.display.focus_text_color affects the text color.

Although you can't change the focus ring color, you can simulate a focus ring with a color of your choosing by applying a user style.

1. Install the Stylish extension.
https://addons.mozilla.org/firefox/addon/stylish/

2. Save the following as a new style.

@namespace url(http://www.w3.org/1999/xhtml);

a:focus {
  border: 2px dashed red !important
}

The thing is, firefox 3.6 was much better on this part. Every link clicked had a box around it and with the color of the text. There was no need to change browser.display.focus_ring_on_anything to true, and therefore no point in changing, adding styles.

I have yet to try the stylish addon.

Edit: tried, and doesn't work as expected.

Again I say it was much better on 3.6

1. I know previous versions didn't have this issue. Pointing out the relevant bug report is the first thing I did. Since the problem has been reported, it may be fixed at some point. Nothing more can be done. Firefox 3.6 has been dead since April 24th 2012 and no one here can change that.

2. I then spent more than a little effort providing two solutions.

3. If you're not going to detail what you don't like about the user style solution, then I can provide no more suggestions.

Can it somehow be made to show a dotted box 1px thick, with the color of the link. and not only red.

@namespace url(http://www.w3.org/1999/xhtml);

a:focus {
  border: 1px dotted blueviolet !important
}

If you want to change it further, see the following link:

• https://developer.mozilla.org/en-US/docs/CSS/border

OK

@namespace url(http://www.w3.org/1999/xhtml);

a:focus {

 border: 1px dotted

}

seems to do the trick.

now, how to make the default focus ring to not appear