It depends on which web browser you are using.
If you are using Safari, right click, or hold down the 'control' key and click on the link to the text file and choose "Download linked file as ..."
If you are suing Chrome or Firefox, click on the link to the text file and from the "File" menu choose "Save page as ..."
If you are using Internet Explorer, right click on the link to the text file and choose "Save target as"
The FIMO text output is not a FASTA file. It's a tabular format that's similar to BED format. You'd have to write your own script to convert from the FIMO text output to BED format. Note though that FIMO also provides its output in GFF format, which is also very similar to BED format. You may want to download that instead. I believe there are Galaxy tools for converting from GFF to BED format, but that's really more of a question for the Galaxy documentation.