A Firefox update will certainly not be intended to change your homepage.

I am not sure what you mean by sidebars. I do not recall any change of the Firefox History or Bookmarks sidebars. Are the sidebars you refer to related to some Firefox extension you have installed ? if so what is that ?

These are the Firefox 40 Release notes

• https://www.mozilla.org/en-US/firefox/releases/ =>
  • Fx40.0 (Developers + )

There are not really any major changes to layout, or anything I think is likely to cause "changes it made to my computer"

Sorry if I am not being very helpful, but if you can provide a screenshot or some more detail about what has changed I will try to help. I don't know whether for instance you may have many extensions and heavy customisations.

It is possible to revert to old versions but I would strongly recommend against trying that. Most versions contain security fixes, and once the bad guys are aware of the fixes they may take advantage of those that remain on or downgrade to unsafe versions. I you really want to and are aware it increases risks to your computer and your data

• (Don't) Install an older version of Firefox <- it has the required links and information
  • Security Fixes you mis out on: https://www.mozilla.org/en-US/security/known-vulnerabilities/firefox/

You can find the home page setting here:

• Tools > Options > General > Startup: Home page

• https://support.mozilla.org/kb/How+to+set+the+home+page

Firefox supports multiple home pages separated by '|' (pipe) symbols.

If you do not keep changes after a restart or otherwise have problems with preferences, see:

• http://kb.mozillazine.org/Preferences_not_saved

What sidebar are you talking about?

Is that the bookmarks sidebar?

• View > Sidebar > Bookmarks (Ctrl+B)
• Navigation Toolbar: "Show your bookmarks" > View Bookmarks Sidebar