It took quite some while, but final it is there: the bidirectional synchronization between Google Calendar and Mozilla Thunderbird. I know, synchronization should be bidirectional by default, but lately the word synchronization was used with different meanings. However, I do not care anymore - it finally works!
I have tried it my own. WITH offline support!!! So, you can update your Google Calendar via Thunderbird and everything you change in Google Calendar will also show up in Thunderbird and can be displayed off-line (you have to choose “Work offline” from the “file” menu though). You can also make changes to Thunderbird and it will show up in Google Calendar after a while.
This made my day. I was waiting for this so long. 
The respective links:
It might have some bugs still, but so far it worked fine for me.
You might want to read the following posts as well:- Spicebird - Information & Communication Center
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- FSC si1520 and Ubuntu 7.04 “Feisty Fawn”
- Google Gears does not work with Firefox on Ubuntu 7.04
28 Nov 07
12:41 am
[...] previous post “Bidirectional syncronization between Google Calendar and Mozilla Thunderbird” was written with a Firefox-Plugin called “Deepest Sender“. From the first test I [...]
28 Nov 07
1:26 am
Cool! That’s the first thing I will try tomorrow morning!
I really like Thunderbird and Lightning though I was scared of it when I was little
but now…it might be a reason to love it 