Unable to see it on my WinXP XAMPP install and my Debian Linux install of the latest snapshot.

Apache 2.4.3 ?
I've tested it on WinXP-XAMPP and Debian Squeeze with Apache 2.2+ and PHP 5.3.x on WindowsXP/7/8.1 clients
Please use the FA v2.3.24+ snapshot instead of the release version and see if that makes a difference.

Does this have anything to do with the LinuxMint GUI on the client side? Check with FireFox versions like 37.0.2 (after which the http port on the client PC was opened up for the browser) or older.

Even though the insert.js is the one controlling the keystrokes and their intended functionality, it is possible that the underlying OS (LinuxMint) does not allow it's international functionality and controls to be overridden.

I've tried in Windows 7 and got the same problem when I open the demo site (http://demo.frontaccounting.eu). Let's make this more clear. Here are the steps to reproduce the problem:
1. Login to FrontAccounting
2. Click on any tab (eg. Items and Inventory)
3. Press Alt + Tab to navigate through the opened windows and back again to FrontAccounting page
4. Click on another tab (eg. Manufacturing) and there will be no respond.

If we are entering data on a form, we can still continue the input process and submit it. We only can't click another link to move to another page after we press Alt + Tab and navigate to another window.

NOTE: If I press "Alt" (just Alt button) once again or press F5, the link is just working normally and I can go to another page.

I don't know if it's only me who experience this, but I will try what you have suggested. I'll try the snapshot version.

Thank you.

Does this have anything to do with the LinuxMint GUI on the client side? Check with FireFox versions like 37.0.2 (after which the http port on the client PC was opened up for the browser) or older.

Even though the insert.js is the one controlling the keystrokes and their intended functionality, it is possible that the underlying OS (LinuxMint) does not allow it's international functionality and controls to be overridden.

FA v2.3.24+ works fine with no fix done. Tested on FireFox 34+ and XAMPP 1.7+ on standard default theme.

What browser and what server and client OSes did you test it on and what themes needed it?

Is it that you are using a very old version of FA.

The entire revision set history for the said file from v2.2.1 onwards (Dec 2009) till date version wise is here.

The history from Aug 2008 on a commit-wise basis is here.

Is this raking up this old post?

Hey, that's really surprising me because I'm using FA v2.3.24 with default theme too. Here are my software specification when I tested it.
- Browser : Firefox 40, Google Chrome 44
- OS : Linux Mint 17.2
- Web server : Apache 2.4.3
- PHP : 5.4.7
- MySQL : 5.5.27

What browser and what server and client OSes did you test it on and what themes needed it?

Is it that you are using a very old version of FA.

The entire revision set history for the said file from v2.2.1 onwards (Dec 2009) till date version wise is here.

The history from Aug 2008 on a commit-wise basis is here.

Is this raking up this old post?

I made the fix in inserts.js file based on the following link after searching on Google:
https://frontaccounting.cvs.sourceforge.net/viewvc/frontaccounting/frontaccount/js/inserts.js?r1=1.6&r2=1.7

To reproduce this problem:
Navigate through every windows with Alt + Tab and go back to FrontAccounting page.

Here is the file. Don't forget to clear the cache before trying it.
https://drive.google.com/file/d/0B4yGh9VGW9V4bzRuSkczN2J5NjQ/view?usp=sharing