Make sure that you do not use Clear Recent History to clear the Browsing History when you close Firefox.

• https://support.mozilla.com/kb/Clear+Recent+History
• https://support.mozilla.com/kb/Firefox+does+not+ask+to+save+tabs+and+windows+on+exit

Firefox 4 and later versions save the previous session automatically, so there is no longer need for the dialog asking if you want to save the current session.
Use "File > Exit" or "Firefox > Exit" (Mac: "Firefox > Quit Firefox") if you want to restore multiple windows.
You can use "History > Restore Previous Session" to get the previous session at any time.
You may need to click the orange/gray Firefox button to see History.
There is also a "Restore Previous Session" button on the default about:home Home page.

I do not have Clear Recent History set.

Restore Previous Session is gray.

I can get the previous session back if I go to History | Recently Closed Windows > and select the most recently closed window.

I think it is not supposed to work this way.

Here is my troubleshooting info:

 Application Basics

       Name
       Firefox

       Version
       5.0

       User Agent
       Mozilla/5.0 (Windows NT 5.1; rv:5.0) Gecko/20100101 Firefox/5.0

       Profile Directory

         Open Containing Folder

       Enabled Plugins

         about:plugins

       Build Configuration

         about:buildconfig

 Extensions

       Name

       Version

       Enabled

       ID

       AVG Safe Search
       10.0.0.1387
       true
       {1E73965B-8B48-48be-9C8D-68B920ABC1C4}

       Java Console
       6.0.21
       true
       {CAFEEFAC-0016-0000-0021-ABCDEFFEDCBA}

       Java Console
       6.0.23
       true
       {CAFEEFAC-0016-0000-0023-ABCDEFFEDCBA}

       Java Console
       6.0.24
       true
       {CAFEEFAC-0016-0000-0024-ABCDEFFEDCBA}

       Java Console
       6.0.26
       true
       {CAFEEFAC-0016-0000-0026-ABCDEFFEDCBA}

       Java Quick Starter
       1.0
       true
       jqs@sun.com

       Microsoft .NET Framework Assistant
       1.2.1
       false
       {20a82645-c095-46ed-80e3-08825760534b}

 Modified Preferences

     Name

     Value

       accessibility.typeaheadfind.flashBar
       0

       browser.history_expire_days.mirror
       20

       browser.places.smartBookmarksVersion
       2

       browser.startup.homepage
       about:blank

       browser.startup.homepage_override.buildID
       20110615151330

       browser.startup.homepage_override.mstone
       rv:5.0

       extensions.lastAppVersion
       5.0

       general.useragent.extra.microsoftdotnet
       ( .NET CLR 3.5.30729)

       gfx.blacklist.direct2d
       2

       gfx.blacklist.layers.direct3d10
       2

       gfx.blacklist.layers.direct3d10-1
       2

       network.cookie.prefsMigrated
       true

       places.database.lastMaintenance
       1310002304

       places.history.expiration.transient_current_max_pages
       31940

       places.last_vacuum
       1306370526

       print.print_printer
       EPSON Stylus CX7400 Series - Network

       print.printer_EPSON_Stylus_CX7400_Series_-_Network.print_bgcolor
       false

       print.printer_EPSON_Stylus_CX7400_Series_-_Network.print_bgimages
       false

       print.printer_EPSON_Stylus_CX7400_Series_-_Network.print_command

       print.printer_EPSON_Stylus_CX7400_Series_-_Network.print_downloadfonts
       false

       print.printer_EPSON_Stylus_CX7400_Series_-_Network.print_edge_bottom
       0

       print.printer_EPSON_Stylus_CX7400_Series_-_Network.print_edge_left
       0

       print.printer_EPSON_Stylus_CX7400_Series_-_Network.print_edge_right
       0

       print.printer_EPSON_Stylus_CX7400_Series_-_Network.print_edge_top
       0

       print.printer_EPSON_Stylus_CX7400_Series_-_Network.print_evenpages
       true

       print.printer_EPSON_Stylus_CX7400_Series_-_Network.print_footercenter

       print.printer_EPSON_Stylus_CX7400_Series_-_Network.print_footerleft
       &PT

       print.printer_EPSON_Stylus_CX7400_Series_-_Network.print_footerright
       &D

       print.printer_EPSON_Stylus_CX7400_Series_-_Network.print_headercenter

       print.printer_EPSON_Stylus_CX7400_Series_-_Network.print_headerleft
       &T

       print.printer_EPSON_Stylus_CX7400_Series_-_Network.print_headerright
       &U

       print.printer_EPSON_Stylus_CX7400_Series_-_Network.print_in_color
       true

       print.printer_EPSON_Stylus_CX7400_Series_-_Network.print_margin_bottom
       0.5

       print.printer_EPSON_Stylus_CX7400_Series_-_Network.print_margin_left
       0.5

       print.printer_EPSON_Stylus_CX7400_Series_-_Network.print_margin_right
       0.5

       print.printer_EPSON_Stylus_CX7400_Series_-_Network.print_margin_top
       0.5

       print.printer_EPSON_Stylus_CX7400_Series_-_Network.print_oddpages
       true

       print.printer_EPSON_Stylus_CX7400_Series_-_Network.print_orientation
       0

       print.printer_EPSON_Stylus_CX7400_Series_-_Network.print_pagedelay
       500

       print.printer_EPSON_Stylus_CX7400_Series_-_Network.print_paper_data
       1

       print.printer_EPSON_Stylus_CX7400_Series_-_Network.print_paper_height
       11.00

       print.printer_EPSON_Stylus_CX7400_Series_-_Network.print_paper_size_type
       0

       print.printer_EPSON_Stylus_CX7400_Series_-_Network.print_paper_size_unit
       0

       print.printer_EPSON_Stylus_CX7400_Series_-_Network.print_paper_width
       8.50

       print.printer_EPSON_Stylus_CX7400_Series_-_Network.print_reversed
       false

       print.printer_EPSON_Stylus_CX7400_Series_-_Network.print_scaling
       1.00

       print.printer_EPSON_Stylus_CX7400_Series_-_Network.print_shrink_to_fit
       true

       print.printer_EPSON_Stylus_CX7400_Series_-_Network.print_to_file
       false

       print.printer_EPSON_Stylus_CX7400_Series_-_Network.print_unwriteable_margin_bottom
       0

       print.printer_EPSON_Stylus_CX7400_Series_-_Network.print_unwriteable_margin_left
       0

       print.printer_EPSON_Stylus_CX7400_Series_-_Network.print_unwriteable_margin_right
       0

       print.printer_EPSON_Stylus_CX7400_Series_-_Network.print_unwriteable_margin_top
       0

       privacy.sanitize.migrateFx3Prefs
       true

       security.warn_viewing_mixed
       false

 Graphics

       Adapter Description
       Intel(R) 82945G Express Chipset Family

       Vendor ID
       8086

       Device ID
       2772

       Adapter RAM
       Unknown

       Adapter Drivers
       igxprd32

       Driver Version
       6.14.10.4764

       Driver Date
       1-13-2007

       Direct2D Enabled
       false

       DirectWrite Enabled
       false (0.0.0.0, font cache n/a)

       WebGL Renderer
       (WebGL unavailable)

       GPU Accelerated Windows
       0/2

Well, now the option to start with windows and tabs from last time seems to be working.

All I did was reboot. I wonder if that was the fix?

Are you using "Show my windows and tabs from last time" ?

• Tools > Options > General > Startup: "When Firefox Starts": "Show my windows and tabs from last time"

Do you use File > Exit Firefox to close all open windows at once?

If not then Firefox will only restore the last closed window.

Sorry, but I did mention in my original post that "have always set my options to open Firefox with the tabs from my previous session". I never use File>Exit. I always click on the red "X" at the top-right of the window. I re-installed Firefox 5.0 and it seems to have cleared up this problem. FF, however is still acting odd. I am running a Windows 7 machine, and have FF on my quick start icons on the bottom-left of my screen. It used to be that once I had started up FF, this icon was highlighted. Now, Windows 7 actually creates a second FF icon at the right end of the quick start icons. However, when I exit (using the X to close the window), it exits but when I re-start FF, my tabs are there again, so I appear to be OK. The other odd thing when I had the problem was that there was absolutely nothing in the History tab, so your original idea to go back to History to get my past sites open in tabs wouldn't work. That's when I tried the re-install.