Ok try to remove both (z4root & root explorer) from your device and reboot
make sure you have copy khmer os.ttf & limon S1.ttf file to your tab.
1. Install Z4Root but do not run it yet
2. Install Root Explorer
3. Run Z4Root and root your device as temporary root when you will see a message that your device is rooted until reboot (do not reboot your tab otherwise your tab will unrooted)
4. Run Root Explorer go to folder where khmer os & limon s1 font is stored then touch and hold on one of them (in my example I touch and hold on khmer os.ttf then select copy)
Go back to root folder then system/fonts touch on mount R/W then paste the font to this folder
Delete an unused font DroidSanSArabic.ttf then rename khmer os.tff to DroidSanSArabic.tff
5. Repeat step 4 to overwrite limon s1.ttf with DroidSansThai.tff
6. Download Mozilla Firefox from the market and enjoy your web surfing
I have done this with own tab and friends' tab with a issue.