adblockfilters

Why does this extension do only half of the job? Automatic download is missing!

Adblock Plus can already download a filter list periodically by itself.
Telling Adblock Plus to do so is as easy as opening the options window
(clicking the arrow of the toolbar button or right clicking the status bar icon)
and opening the dialog for adding a new filter subscription from the "Filters" menu.
In this dialog you give the subscription a name of your choice (e.g. "My Own")
and give the URL you're uploading the filters to.

Is there any way to synchronize two Adblock Plus installations?

Not at this point. This is on the to do list, but it will require some fair amount of work. For now, you have to

If all you want, however, is to synchronize ABP in different operating systems on a single computer, you can set the extensions.adblockplus.patternsfile preference accordingly.

If you really need synchronization, you need it now, and you don't mind some serious problems, you can get the Filter Admin add-on additional to the Filter Uploader from CVS. But be warned that this is only a half-hearted solution and there's no support or documentation for this. You'll have to figure out how to make it work for you by yourself.

Where should I upload the filters to?

If you don't already have a hosting provider, there are many free ones.
Just google for them...

What should I put into the Host and Path fields?

This depends on your web hoster. However, most follow some conventions.
Given your files were available at http://www.example.com/yourusername/, the host
will most likely be either example.com or ftp.example.com and the path will be
/yourusername/adblock.txt or just /adblock.txt. In some cases, the files that
are available via http will also be under a directory called /www/ or similar.

Note that the host isn't preceeded
by "ftp://" or "http://" nor has it a trailing slash.

Also, the path should begin with a slash and it MUST contain the filename.
If you're just providing a folder name, this will lead to a error message that's hard to interprete.

Is there a way to include the modification date in the file?

Yes, from version 1.7 on, the string $DATE in a comment is replaced with date and time of the upload. So you can add this comment, for example:

! Last Modified: $DATE

On a German Ubuntu this leads to this output:

! Last Modified: Mo 09 Feb 2009 19:21:06 CET

Why can't I enable gzip compression?

Gzip compression is only supported in Gecko 1.9 (Firefox 3) applications and higher.
In older versions, gzip compression is not available.

How do I have to configure my web server so I can use gzip compression?

This generally depends on your server. In Apache you can put e.g. this in your httpd.conf or .htaccess:
(given that the filename is adblock.txt.gz)

<Files adblock.txt.gz>
    AddType text/plain .gz
    AddEncoding gzip .gz
    AddCharset UTF-8 .gz
</Files>

For other servers, Google should know the answer.