Well I don't know what add-ons you have, so I can't say what you'll lose. You ma not lose any. Just try the update and see what happens :) Also, your user agent is corrupt. How to reset the default user agent on Firefox

please note that continuing using an outdated versions of firefox like 3.6.28 which is affected by numerous known security vulnerabilities will put the integrity of your system and all the personal data on it at risk! so I'd recommend updating as soon as possible.

in order to answer your specific question it would be necessary to know which addons you've installed specifically - google toolbar won't work any longer but there are quite a few alternatives: Google Toolbar is no longer compatible with Firefox - Here are some alternatives

regarding your mouse/standby issue (which should not be related to firefox) see www.howtogeek.com/howto/15132/stop-your-mouse-from-waking-up-your-windows-7-computer/

Extensions if they have not been updated when using 3.6.* all this time will likely need to be updated.

Most extensions that were compatible with Firefox 4.0 should work unless.

- Extensions which use XPCOM binary components must be recompiled specifically for the Firefox version. (antivirus toolbars for example)

- Extensions that use the "strict compatibility" flag in the install.rdf.

- Extensions with a max version compatibility lower than Firefox 4.0.

- Extensions tested by addons.mozilla.org and discovered to be incompatible by automated testing.

Also Themes which must be specifically updated to support new features.