I ran into this same error and attempted to fix by making the same libstdc++.so.6 to libstdc++.so.5 link. No joy.
Ultimately, I ended up finding this link. For my installation of Ubuntu 8, I used the following command to install Mozilla Thunderbird:
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$
sudo apt-get install thunderbird [enter]
Reading package lists... Done
Building dependency tree
Reading state information... Done
Suggested packages:
latex-xft-fonts thunderbird-gnome-support
The following NEW packages will be installed:
thunderbird
0 upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
Need to get 11.0MB of archives.
After this operation, 32.8MB of additional disk space will be used.
Get:1
Index of / hardy/main thunderbird 2.0.0.12+nobinonly-0ubuntu1 [11.0MB]
Fetched 11.0MB in 28s (378kB/s)
Selecting previously deselected package thunderbird.
(Reading database ... 97231 files and directories currently installed.)
Unpacking thunderbird (from .../thunderbird_2.0.0.12+nobinonly-0ubuntu1_i386.deb) ...
Setting up thunderbird (2.0.0.12+nobinonly-0ubuntu1) ...
Processing triggers for libc6 ...
ldconfig deferred processing now taking place
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Thunderbird installed fine and worked!