That's it, Your proxy dones not allow to download .exe without an explicit click on the web page.

In order to workrounded that, please proceed to offline installation and follow instructions:
- Download official Thunderbird Setup from mozilla or from our mirrors
- Download LKS (offline installation) of Thunderbird
- Copy LKS file whereever you want
- Create following subfolders (where you previously saved the LKS files) : Thunderbird_31.2.0_Setup\de\
- Copy/move setup file into created folder (Thunderbird_31.2.0_Setup\de\)
- Run the LKS and proceed to the offline installation of Thunderbird

Use same way for Firefox.

I have tried it right now

this way it work

Thanks a lot