im a claws mail man ; using bogofilter for spam which gets email from gmail. Last time i had to set iy up theres a handy function to re-create tree file structure that is on gmailThunderbird and Seamonkey are good email client.
Seamonkey is an browse and email client. The good detail is that use much less RAM than Firefox and is possible change several settings in propertis where Firefox not allow being only possible using about:config.
Using Thunderbird and using internet all time you need FIrefox and Thunderbird in RAM and will use more RAM. Seamonkey is both browse and email client and will use less RAM and the only problem is some few sites displaying false warning that the browse not is Firefox being easy to fix it.
When changing OS or formating even saving Thunderbird or Seamonkey email folder not is simple to copy to new installed system if you not copy totally the profile folder and if you download all email to computer and deleted in email server caution you will have problems that can be fixed if installing plugin importexporttools that you select the previous email folder and copy totally to new email folder.
Need say again ... caution if downloaded all emails and deleted all in email server and done an partition format.