If you are using two computers and if you want to export / copy / transfer / send the Firefox extensions from 1 computer to other computer, you can follow the following steps :
1) Download and install FEBE extension to your Firefox.
2) Restart your Firefox.
3) On your Firefox, click Tools > FEBE > FEBE Options > Directory > Change the directory to your desktop, so that all the extension backup files will be saved on your desktop later.
4) On your Firefox, click Tools > FEBE > Perform Backup.
5) Now you see all the extension backup files are on your desktop.
6) Send all the extensions backup files to your other computer.
7) Open Firefox on your other computer, click File, click Open File, select the extension backup file.

Note that you must open 1 by 1 of the extension backup file to restore all the extensions onto the Firefox of other computer. If you have more than 30 extensions backup files, you will need to repeat the same tasks for 30 times!!! It is very inconvenient. Actually you can use a software to combine all the 30 extension backup files into 1 backup file. Steps :
1) Download and install CLEO extension to your Firefox.
2) Restart your Firefox.
3) On your Firefox, click Tools > FEBE > CLEO > CLEO Options > Directory > Change the directory to your desktop, so that the extension backup files will be saved on your desktop later.
4) On your Firefox, click Tools > FEBE > Create Cleopack > Select items to package > Select all the extension backup files that you have created by FEBE just now.
5) Now you see the cleopack file is on your desktop.
6) Send the cleopack file to your other computer.
7) Open Firefox on your other computer, click File, click Open File, select the cleopack file.