Yes, that is a problem with that extension (has been reported by others).
If there isn't an update available that fixes this then you need to disable or uninstall that extension.

• Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

Thanks for that, I wasn't aware that it had been reported by others. I'll await an update for the extension or remove it...

Whether the cursor goes to the Search Box or to the Address Box when you open a new tab is based on which page your browser opens when you open a new tab. If your browser is set to open http://en-US.start3.mozilla.com/firefox?client=firefox-a&rls=org.mozilla:en-US:official when a new tab is opened, the cursor will automatically go to the Address Bar. If your browser is set to open http://www.google.com/firefox?client=firefox-a&rls=org.mozilla:en-US:official when a new tab is opened, the cursor will automatically go to the Search Box. So, it just depends on which webpage opens when you select a new tab! To select which webpage opens, just go to the "Tools" Menu and select "Options." Under the "General" tab, you will see a URL (a webpage) which will open when you open your Home Page. I suppose, technically, the webpage you select in "Tabs Mix Plus" to open when you open a new tab is the important one -- but, since most people simply use their Home Page as the page to open, the Home Page in the Firefox Options is relevant. You could change the webpage to open in Tabs Mix Plus to one of the two pages noted above to determine which option you want when you open a new tab -- to have the cursor in the Address Bar or to have the cursor in the Search Box.

Little nuances such as this annoy me, as does the layout of all Firefox versions after 3.x. So, I gave Chrome a chance, as it is not too far off from the present Firefox. When I applied this extension, https://chrome.google.com/webstore/detail/icpgjfneehieebagbmdbhnlpiopdcmna it worked the same way Firefox used to.

Although it may be quite pious to offer a different browser as a solution, especially in a Firefox forum, it seems to be the logical remedy to a problem. Ask yourself a question, how much coding is really involved in allowing users to open a new tab and already have it filled with your search engine or home-page of choice? If a donation would make this happen, I would proudly do just that.

End of line.