Regarding to fix, seems like another issue has come out. Now if you swicth between signatures the preview is fine updated, until you make a modification to one of them. Then neither the edit text box nor the preview are updated.

My expected behaviour should be a warning dialog box saying you haven't save the modifications and asking you if you wanna really switch without saving them. Pressing OK should update both and the email itself too (this is actually working right now) without saving and pressing Cancel should keep everything outdated even the email and returning the drop down menu to its previous state.

Aplicación: Firefox 3.6 (20100115144158)
Sistema Operativo: WINNT (x86-msvc) --> Windows 7 Ultimate (x86)

- About Menu 1.3.2.1
    (Disabled, Incompatible)
- about:me 0.4.1
- Add-on Collector 1.0.4
- Add-on Compatibility Reporter 0.4.2
    (Disabled)
- AutoPager 0.6.0.14
- AutoSaveText 1.33
    (Disabled, Incompatible)
- AVG Safe Search 9.0.0.716
- AVG Security Toolbar 3.011.025.005
- Better Amazon 0.1
- Better Flickr 0.4.1
- Better GCal 0.3
    (Incompatible)
- Better Gmail 2 0.9.8
- Better GReader 0.8.3
- Better YouTube 0.4.3
- Blank Canvas Gmail Signatures 1.12.06b
- Bookmark Autohider 0.1.7.1
    (Disabled)
- ChromaTabs Plus 2.3
- ChromeBug 1.6.0a5
    (Disabled)
- CLEO 4.3
- CodeBurner for Firebug 1.5.2
    (Disabled)
- CodeBurner for Firefox 1.0
    (Disabled, Incompatible)
- ColorfulTabs 4.3
    (Disabled)
- Cooliris 1.11.6
- Copy Link Text 1.4.4
- Coral IE Tab 1.70.20100204
- CSS Usage 0.2.1
    (Disabled)
- Ctrl-Tab 0.21
    (Disabled)
- Custom Buttons 0.0.4.6
    (Disabled)
- DAEMON Tools Toolbar 1.1.1.0014
    (Disabled)
- Default Full Zoom Level 4.3
- Define 1.3.0
    (Disabled)
- Delicious Bookmarks 2.1.072
    (Disabled)
- Download Panel 2009.09.02
    (Disabled)
- Download Statusbar 0.9.6.7
- Echofon 1.9.5.1
    (Disabled)
- Extension List Dumper 1.14.8
- Extension Manager Extended 2.8.1
- FabTabs 1.3.4.1
    (Disabled)
- Faviconiac Search Engines 1.09
    (Disabled, Incompatible)
- FEBE 6.3.2
- Find In Tabs 0.4.0.6
    (Incompatible)
- FireAtlas 1.1.25
    (Disabled)
- Firebug 1.5.2
    (Disabled)
- Firebug Net Panel History Overlay 0.3.0
    (Disabled, Incompatible)
- FirebugCodeCoverage 0.1
    (Disabled, Incompatible)
- Firecookie 1.0.2
    (Disabled)
- FireDiff 1.0a1
    (Disabled, Incompatible)
- Firefinder for Firebug 1.01
    (Disabled)
- Firefocus 1.0
    (Disabled, Incompatible)
- Firefox Showcase 0.9.5.5
- Firefox Synchronisation Extension 7.3.2.22
- FirePalette 0.1.6
    (Disabled, Incompatible)
- FirePHP 0.4.3
    (Disabled)
- Firepicker 1.3.1
    (Disabled)
- FireQuery 0.6
    (Disabled)
- FireRainbow 1.0
    (Disabled, Incompatible)
- FireShot 0.81
- FireShot 0.80
    (Disabled)
- Firesizer 1.0
- Flagfox 4.0.2
- FlashGot 1.2.1.15
- FootieFox 2.1.8
- FoxClocks 2.5.46
- Glasser 3.5.2
- Gmail Notifier 0.6.4.1
    (Disabled)
- Google Bloc de Notas 1.0.0.22
    (Disabled, Incompatible)
- Google Gears 0.5.33.0
    (Disabled, Incompatible)
- Greasefire 1.0.4
- Greasemonkey 0.8.20100211.5
- Hide Find Bar 1.3.1
- HighlightAll 1.4
    (Disabled)
- Incredible Bookmarks 0.7.1
    (Disabled)
- InFormEnter 0.5.5.8
- Inline Code Finder for Firebug 1.0
    (Disabled)
- Intelligent Middle Clickums 0.0.2
    (Disabled)
- It's All Text! 1.4.1
    (Disabled)
- Java Console 6.0.18
- Jetpack 0.8.1
    (Disabled)
- Jiffy 1.1.20090709
    (Disabled, Incompatible)
- LiveCoder 0.4
    (Disabled, Incompatible)
- Locationbar Limit 2008.11.17
- Locationbar² 1.0.4
- MeasureIt 0.3.92
    (Disabled)
- Menu Editor 1.2.6
    (Disabled)
- Microsoft .NET Framework Assistant 1.1
    (Disabled)
- Move Media Player 1.0.0.071301000019
    (Disabled)
- Multiple Tab Handler (Manipulador múltiple de Pestañas) 0.5.2010020801
- myBabylon English Toolbar 2.5.6.0
    (Disabled)
- Nuke Anything Enhanced 1.0.2
    (Disabled)
- OneClick YouTube Downloader 1.0.4
    (Disabled)
- Open in Browser 1.7
    (Disabled)
- Open In RegEdit 0.1.2.4
    (Disabled)
- OpenInWindow 1.2.1
    (Disabled)
- OPIE 1.2.3
- Ovi maps browser plugin 2.3.37.6
    (Incompatible)
- Personal Menu 4.2.1
- Personas 1.5.1
- Pixelpipe 1.3
    (Disabled)
- Prism for Firefox 1.0b3
    (Disabled)
- QuickPrint 0.1.1
- QuickRestart 1.1.6
- Rainbow 0.9
    (Disabled)
- Read It Later 2.0.3
- Reframe It 2.0.0.20091223
    (Disabled)
- Reload Plus 1.0
    (Disabled)
- ReloadEvery 3.6.2
    (Disabled)
- Resizeable Textarea 0.1d
- Save File to 2.0
- Save Image in Folder 1.3.1
    (Disabled)
- Save Link in Folder 1.5.2
    (Disabled)
- searchOnTab 1.0.3
- SearchPreview 4.1
- Shrunked Image Resizer 2.0.1
    (Disabled)
- Sign That Sis! 0.2
    (Disabled, Incompatible)
- Simple Timer 1.7.1
- SkipScreen 0.3.20091214_AMO
- Skype extension for Firefox 4.2.0.5198
    (Disabled)
- Smiley Xtra 5.0.2
    (Disabled)
- Source Viewer Tab 0.3.2009090201
    (Disabled)
- Split Browser (Dividir navegador) 0.6.2009110501
    (Disabled)
- Stop Autoplay 0.7.6
    (Disabled)
- Tab Progress Bar 0.6
- Tab Scope 0.3
    (Disabled)
- TabCloser 1.06
    (Disabled)
- TabCounter 1.0.3
- Table2Clipboard 1.3
    (Disabled)
- TabRenamizer 0.9.6
    (Disabled)
- Test Pilot 0.4
- ThumbnailCopy 1.0.1
    (Disabled, Incompatible)
- TimeTracker 1.2.5
- Toggle Private Browsing 1.8
    (Disabled)
- Togle Menubar 1.1.0
    (Disabled, Incompatible)
- TooManyTabs 1.1.5
- Ubiquity 0.5.4
    (Disabled, Incompatible)
- Unicode Input Tool/Converter 2.6.4
    (Disabled)
- URL Link 2.03.3
- UrlbarExt 1.6.1
- User Agent Switcher 0.7.2
- ViewAbout 1.5
- Weave Sync 1.0.1
- WiseStamp 1.3.5
    (Disabled)
- Wolfram Alpha Google 1.0
- XRefresh 1.3
    (Disabled)
- YSlow 2.0.6
    (Disabled)