I have found how to use Client Certificates with Android OS:

1. Install and run at least once Mozilla Firefox or Mozilla Fennec for Android .

2. Please download from your Android phone to the PC: - Mozilla Firefox:

/data/data/org.mozilla.firefox/files/mozilla/.default/cert9.db

/data/data/org.mozilla.firefox/files/mozilla/.default/key4.db

- Mozilla Fennec:

/data/data/org.mozilla.fennec/files/mozilla/.default/cert9.db

/data/data/org.mozilla.fennec/files/mozilla/.default/key4.db

3. Move them to a MS Windows directory (Ex.: C:\keys )

4. Download the package NSS_Tools_x86_from_NSS_3.12.7 Tools.zip and extract it into a directory (Ex.: c:\nss-3_12_7)

5. Run command prompt ( CMD.EXE ) and change the directory where you have extracted NSS_Tools_x86_from_NSS_3.12.7 Tools.zip(Ex.: "cd c:\nss-3_12_7")

6. Executhe the command:

 pk12util.exe -i  -d sql:  ( Ex.: c:\nss-3_12_7>pk12util.exe -i c:\epay.p12 -d sql:C:\keys )    Enter password for PKCS12 file:   pk12util.exe: PKCS12 IMPORT SUCCESSFUL 

 If you have more client certificates - do the same command again. 

7. Move the "cert9.db" and "key4.db" files back to your Android phone. If necessary fix the ownership and access rights.

8. Restart Mozilla Firefox or Mozilla Fennec for Android.

9. If you access a web site that needs client certificate authentication the browser will ask you to choose one of the imported client certificates and will you them

Final words: Now even on Android platform it is possible to use client certificates for SSL authentication and signing. If there was an add-on or a setting to manage them it would be much easier. I hope that Fennec developer team will fix this issue in the near future. It is also possible to use PKCS#11 library for accessing secure signature devices plugged into themicroSD slot and this will make possible to use Qualified digital certificates with Android OS.