The Addon does the job, but the workflow with the address bar is far from perfect.
When you use "custom url" the addon makes the address bar unusable for the time it takes the browser to completely load the page defined in "custom url".


Let's say my "custom url" is "A".
I open the new tab and start typing the address "B" in the address bar (different from custom url).

1) If the page "A" loads before I finish typing url "B" all my typing was useless, because the addon overwrites my typed url "B" with "A". Or concatenates B+A (more often). I have to start over.

2) If I finish typing url "B" quicker then the addon loads the page "A" (custom url) and press Enter, in most cases the browser ignores my actions. And again, the addon overwrites my typed url "B" with "A". Or concatenates B+A (more often). And again I have to start over.

The only way to cope with this task is to hit ESC key right after Ctrl+T combination, to prevent Firefox from loading the page in the new tab.

As some people said before, it would be better, if the addon would leave the clean address bar and wouldn't mess with user typing. Manual URL entering should have priority over "custom url" loading.
If I want the defined "custom url" to load I would wait for the tab to load. But if I type the URL myself, don't reset the address bar, don't overwrite my entered text.

It's just getting on my nerves. But unfortunately my favorite bookmark-site doesn't have the Firefox addon, so I have to use some middleman-addon like New Tab Override. And the process reminds me of the windows 98 setup, where if to paraphrase they would say "sit back and relax while Firefox+NewTabOverride load the page. Then and only then you will be able to use the address bar to go to some other page"

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if you have your CPU heavy loaded with some background task you can see the whole workflow:
Let's say I want to use some "search shortcut" ( "y" for YouTube)
1) I open new tab,
2) quickly type "y trailers" and hit ENTER (to go to YouTube and look for movie trailers)
3) Firefox starts to load YouTube search page
4) "New Tab Override" stops the process and loads the "custom url" instead