Last supported version of Firefox ESR 45.9 here (final update May 2017)
This is a more recent, more secure web browser for OS X 10.7.
Firefox 45.9 ESR .dmg install file here.
https://ftp.mozilla.org/pub/firefox/releases/45.9.0esr/mac/en-US/
Dbl click to download, locate the Firefox 45.9 ESR.dmg on your Mac.
Dbl click on the file and it should put up both a disk drive looking image and a pop up installation window.
Drag the Firefox Application icon into the Application folder shortcut icon in the same install window.
The go to your Applications folder, find Firefox, the launch it!
You may need to update Adobe Flash Player to the last compatible version for Firefox 45.9
You need to use Adobe's Flash Player uninstaller, first, to uninstall the older Adobe Flash Player.
https://helpx.adobe.com/flash-player/kb/uninstall-flash-player-mac-os.html
On the Adobe Flash Player section of the Adobe website, you will need to look for the last Adobe Flash Player version compatible with Mozilla Firefox 45.9.
This is the Adobe Flash Player you are looking for on the Adobe web site.
Adobe Shockwave Flash version 26.0.0.137
Look toward the bottom half of this webpage link.
https://fpdownload.macromedia.com/pub/flashplayer/installers/archive/fp_26.0.0.1 31_archive.zip
Once downloaded on your Mac, dbl-click to open the zIp file
Dbl click to Install the Mac OS version if there is one listed in the archive file (might be a .dmg file).
When you open that Adobe Flash Player archive file, it will create an archive folder.
Dbl click that folder, two other folders will appear.
Click the first folder WITHOUT THE words debug in it!
When that folder opens you are looking for a Mac OS .dmg file with this name
flashplayer26_0r0_131_mac.dmg
Dbl click on this and begin the install procedure from the brownish red Adobe install folder that appears.
Good Luck!