With large attachments, TB seems to be "loading message" over and over.

Question

I'm using TB with as POP with a gmail account. I used to use Outlook. When I receive large (20MB) emails with attachments such as .jpg or videos, it APPEARS that TB keeps going out to get the email from the Internet instead of having it locally on the PC as Outlook used to do. If I have several emails in my inbox of this type clicking on one or the other shows "loading" at the bottom of the screen. Hasn't it already been "loaded" once and stored locally? Also, when "saving attachment" or "save all" it appears that it is "downloading" the file. Again, isn't this already "downloaded" in its entirety?
My laptop is brand new, with Widows 8.1, low end, not a terribly fast processor. Even deleting the email seems to show a "progress bar" as if it is going out to the Internet.
The reason for my concern is that I have a 3GB/mo. account thru AT&T and it just seems like it's loading this data each time rather than just once as POP3 is supposed to do.
Is it all an illusion?
Thanks.

Answer 1

Now I see. I'll paste it here:
Application Basics
Name: Thunderbird
Version: 24.5.0
User Agent: Mozilla/5.0 (Windows NT 6.3; WOW64; rv:24.0) Gecko/20100101 Thunderbird/24.5.0
Profile Folder: Show Folder
(Local drive)
Application Build ID: 20140424091057
Enabled Plugins: about:plugins
Build Configuration: about:buildconfig
Crash Reports: about:crashes
Memory Use: about:memory
Mail and News Accounts
account1:
INCOMING: account1, , (pop3) pop.googlemail.com:995, SSL, passwordCleartext
OUTGOING: smtp.googlemail.com:465, SSL, passwordCleartext, true
account2:
INCOMING: account2, , (none) Local Folders, plain, passwordCleartext
account3:
INCOMING: account3, , (pop3) pop.googlemail.com:995, SSL, passwordCleartext
OUTGOING: smtp.googlemail.com:465, SSL, passwordCleartext, true
Extensions
Lightning, 2.6.5, true, {e2fda1a4-762b-4020-b5ad-a41df1933103}
Important Modified Preferences
Name: Value
browser.cache.disk.capacity: 358400
browser.cache.disk.smart_size.first_run: false
browser.cache.disk.smart_size.use_old_max: false
browser.cache.disk.smart_size_cached_value: 358400
extensions.lastAppVersion: 24.5.0
font.internaluseonly.changed: true
font.name.monospace.el: Consolas
font.name.monospace.tr: Consolas
font.name.monospace.x-baltic: Consolas
font.name.monospace.x-central-euro: Consolas
font.name.monospace.x-cyrillic: Consolas
font.name.monospace.x-unicode: Consolas
font.name.monospace.x-western: Consolas
font.name.sans-serif.el: Calibri
font.name.sans-serif.tr: Calibri
font.name.sans-serif.x-baltic: Calibri
font.name.sans-serif.x-central-euro: Calibri
font.name.sans-serif.x-cyrillic: Calibri
font.name.sans-serif.x-unicode: Calibri
font.name.sans-serif.x-western: Calibri
font.name.serif.el: Cambria
font.name.serif.tr: Cambria
font.name.serif.x-baltic: Cambria
font.name.serif.x-central-euro: Cambria
font.name.serif.x-cyrillic: Cambria
font.name.serif.x-unicode: Cambria
font.name.serif.x-western: Cambria
font.size.fixed.el: 14
font.size.fixed.tr: 14
font.size.fixed.x-baltic: 14
font.size.fixed.x-central-euro: 14
font.size.fixed.x-cyrillic: 14
font.size.fixed.x-unicode: 14
font.size.fixed.x-western: 14
font.size.variable.el: 17
font.size.variable.tr: 17
font.size.variable.x-baltic: 17
font.size.variable.x-central-euro: 17
font.size.variable.x-cyrillic: 17
font.size.variable.x-unicode: 17
font.size.variable.x-western: 17
mail.openMessageBehavior.version: 1
mail.winsearch.firstRunDone: true
mailnews.database.global.datastore.id: 61b3273d-7b24-4ff8-bc33-2f1299efbf8
mailnews.database.global.indexer.enabled: false
network.cookie.cookieBehavior: 2
network.cookie.prefsMigrated: true
places.database.lastMaintenance: 1411654561
places.history.expiration.transient_current_max_pages: 92721
plugin.importedState: true
privacy.donottrackheader.enabled: true
Graphics
Adapter Description: AMD Radeon HD 8210
Vendor ID: 0x1002
Device ID: 0x9834
Adapter RAM: 512
Adapter Drivers: aticfx64 aticfx64 aticfx64 aticfx32 aticfx32 aticfx32 atiumd64 atidxx64 atidxx64 atiumdag atidxx32 atidxx32 atiumdva atiumd6a atitmm64
Driver Version: 13.152.1.3000
Driver Date: 9-25-2013
Direct2D Enabled: false
DirectWrite Enabled: false (6.3.9600.17111)
ClearType Parameters: ClearType parameters not found
WebGL Renderer: false
GPU Accelerated Windows: 0
AzureCanvasBackend: skia
AzureFallbackCanvasBackend: cairo
AzureContentBackend: none
JavaScript
Incremental GC: 1
Accessibility
Activated: 0
Prevent Accessibility: 0
Library Versions
Expected minimum version
Version in use
NSPR
4.10.2
4.10.2
NSS
3.15.4 Basic ECC
3.15.4 Basic ECC
NSS Util
3.15.4
3.15.4
NSS SSL
3.15.4 Basic ECC
3.15.4 Basic ECC
NSS S/MIME
3.15.4 Basic ECC
3.15.4 Basic ECC