How can I export & import firefox addons settings

All preferences are stored inside prefs.js file inside your profile folder, using a javascript syntax:

[...]
user_pref("browser.cache.disk.capacity", 358400);
user_pref("browser.cache.disk.smart_size.first_run", false);
user_pref("browser.cache.disk.smart_size.use_old_max", false);
user_pref("browser.cache.disk.smart_size_cached_value", 358400);
[...]    
user_pref("extensions.grabanddrag-version", "3.2.0");
user_pref("extensions.grabanddrag.button", 1);
user_pref("extensions.grabanddrag.flickon", false);
user_pref("extensions.grabanddrag.grabCursor", "auto");
[...]

Once you shut down Firefox, you may grep all your addon configs and append the lines to your new profile file:

grep extensions. oldprofile\prefs.js >> newprofile\prefs.js

or with just a single addon (assuming you've found it's name):

grep extensions.grabanddrag. oldprofile\prefs.js >> newprofile\prefs.js

To avoid having duplicate entries (it's javascript, so the last entry counts), before you import your settings, you may also purge same config settings from your new profile:

grep -v extensions.grabanddrag. newprofile\prefs.js > newprofile\prefs.js

The -v attribute reverses the way grep works.

This will of course move config only. You need to install the addon separately.

Can you try setting up Firefox Sync?

Firefox Sync is a service that lets you synchronize your bookmarks, history, passwords, add-ons and open tabs with another copy of Firefox – like one on another computer or on a mobile phone. This article briefly explains how it all works and why you'd want to use it.

Firefox Sync syncs your Firefox data (passwords, settings, add-ons) between multiple installations of Firefox. I have not used it to sync add-ons, but the other data is synced seamlessly.