All back-copies are available here: https://ftp.mozilla.org/pub/mozilla.org/thunderbird/releases/ It's a bit daunting at first sight, but select the version you want, your platform (mac/linux/linux-64-bit/windows) and your language and you'll be shown the installer to download. I'd right-click, use "save link as", download it, then double-click (or do whatever your OS needs) the downloaded file to open or run it and extract/install Thunderbird. Which particular add-ons are you having trouble with? I downloaded 24.5 tonight and all of my (too) many add-ons seem fine with it.