You can find the fasta files for each chromosome at ftp://ftp.ncbi.nlm.nih.gov/genomes/Gossypium_hirsutum/Assembled_chromosomes/seq/
It doesn't seem to exist there a single fasta (.fa) file (or compressed .fa.gz file) containing all the chromosomes but you can generate yourself with some commands or shell scripting (note that probably you will need to decompress them previously). See for example https://www.biostars.org/p/127239/ or https://www.biostars.org/p/50154/
Another option is to register in the Cotton Genome Project (http://cgp.genomics.org.cn/page/species/download.jsp?category=hirsutum). They say they provide the file but only for academic research use.
You will not get the whole genome as a single Fasta file. Moreover, a single file of whole genome is usually not possible to process by most of the multimedia laptops.Therefore, I suggest you download each chromosome separately and do your analysis.